docs: fix typo in @Args documentation (#4000)

docs/docs/guides/developer-guide/the-api-layer/index.mdx

@@ -389,7 +389,7 @@ As a general rule, _always_ use the `@Ctx()` decorator to inject the `RequestCon
 
 This is exported by the `@nestjs/graphql` package. It is used to inject the arguments passed to a query or mutation.
 
-Given the a schema definition like this:
+Given a schema definition like this:
 
 ```graphql
 extend type Mutation {