Change zero documentation mentions of genre to genres (#5486)

I've spent 2 hours troubleshooting why none of my music had genre tag.
It was because the single `genre`, without `s` doesn't seem to cover any
good ganre tags... at least it didn't on my opus files

looking at the code:
https://github.com/beetbox/mediafile/blob/7ecd86101ec3344e7fb25bbf781cd7f86646aa9d/mediafile.py#L1669-L2167
i don't honestly know why anyone created the single `ganre` field in the
first place

Author: snejus

docs/plugins/zero.rst

@@ -36,9 +36,10 @@ fields to nullify and the conditions for nullifying them:
 For example::
 
     zero:
-        fields: month day genre comments
+        fields: month day genre genres comments
         comments: [EAC, LAME, from.+collection, 'ripped by']
         genre: [rnb, 'power metal']
+        genres: [rnb, 'power metal']
         update_database: true
 
 If a custom pattern is not defined for a given field, the field will be nulled
@@ -60,4 +61,4 @@ art from files' tags unless you tell it not to. To keep the album art, include
 the special field ``images`` in the list. For example::
 
     zero:
-        keep_fields: title artist album year track genre images
+        keep_fields: title artist album year track genre genres images