docs: fix a/an grammar errors in documentation (#7491)

**Description**:
  This PR fixes minor grammar errors found in the documentation:

  **Documentation fixes:**
  - Fixed "an the" → "the" in storage-vec documentation
- Fixed "an ContractId" → "a ContractId" (ContractId starts with
consonant sound)
  - Fixed "an Contract" → "a Contract"

<!-- CURSOR_SUMMARY -->
---

> [!NOTE]
> Fixes incorrect articles in docs ("an"→"a" and "an the"→"the") across
asset mint, ContractId, and StorageVec pages.
> 
> - **Documentation**:
>   - Correct article usage in:
> - `docs/reference/src/documentation/operations/asset/mint/contract.md`
("a Contract").
> -
`docs/reference/src/documentation/operations/namespace/contract-id.md`
("a ContractId").
> -
`docs/reference/src/documentation/operations/storage/libraries/storage-vec.md`
("the" instead of "an the").
> 
> <sup>Written by [Cursor
Bugbot](https://cursor.com/dashboard?tab=bugbot) for commit
3106eec. This will update automatically
on new commits. Configure
[here](https://cursor.com/dashboard?tab=bugbot).</sup>
<!-- /CURSOR_SUMMARY -->

Co-authored-by: Joshua Batty <[email protected]>

Author: serikssonn

docs/reference/src/documentation/operations/asset/mint/contract.md

@@ -1,6 +1,6 @@
 # Mint to Contract
 
-We can [`mint`](mint.md) and [`transfer`](../transfer/index.md) the asset to an [`Contract`](../../namespace/contract-id.md).
+We can [`mint`](mint.md) and [`transfer`](../transfer/index.md) the asset to a [`Contract`](../../namespace/contract-id.md).
 
 To use the function we must import it.
 

docs/reference/src/documentation/operations/namespace/contract-id.md

@@ -8,7 +8,7 @@ The `ContractId` type is a struct containing a value of a `b256` type.
 {{#include ../../../code/operations/namespace/src/lib.sw:contract_id}}
 ```
 
-Casting between an `ContractId` and `b256` can be done in the following way:
+Casting between a `ContractId` and `b256` can be done in the following way:
 
 ```sway
 {{#include ../../../code/operations/namespace/src/lib.sw:contract_id_cast}}

docs/reference/src/documentation/operations/storage/libraries/storage-vec.md

@@ -30,7 +30,7 @@ In this example we look at how we can retrieve a single value `balance` and how
 
 Writing to storage is similar to [reading](#reading-from-storage). The difference is that we use a different method `.push(value)` and we use the `read` keyword because the implementation reads the length of the vector to determine where to store the value.
 
-In this example we insert a [tuple](../../../language/built-ins/tuples.md) containing an the [`Identity`](../../namespace/identity.md) of the caller and some `id` into the vector.
+In this example we insert a [tuple](../../../language/built-ins/tuples.md) containing the [`Identity`](../../namespace/identity.md) of the caller and some `id` into the vector.
 
 ```sway
 {{#include ../../../../code/operations/storage/storage_vec/src/main.sw:writing_to_storage}}