fix: external node link (#438)

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# Description

wrong url in the external node documentation

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## Additional context

Author: bxpana

content/00.zksync-network/40.tooling/50.external-node.md

@@ -52,4 +52,4 @@ With the ZKsync node, you _can not_:
 
 ## Running the Node
 
-Detailed information on how to run, configure, and maintain the External Node can be found in the [ZKsync Core book](https://matter-labs.github.io/zksync-network/core/latest/guides/external-node/01_intro.html).
+Detailed information on how to run, configure, and maintain the External Node can be found in the [ZKsync Core book](https://matter-labs.github.io/zksync-era/core/latest/guides/external-node/01_intro.html).