docs: Fix grammatical error in part-6-performance-normalization.md (#4816)

Author: BonaventureCJ

docs/tutorials/essentials/part-6-performance-normalization.md

@@ -1298,7 +1298,7 @@ The only action we're handling here always replaces the entire list of users wit
 
 We were already exporting the `selectAllUsers` and `selectUserById` selectors we'd written by hand. We can replace those with the versions generated by `usersAdapter.getSelectors()`.
 
-We do now have a slight types mismatch with `selectUserById` - our `currentUsername` _can_ be `null` according to the types, but the generated `selectUserById` won't accept that. A simple fix is to check if exists and just return early if it doesn't.
+We do now have a slight types mismatch with `selectUserById` - our `currentUsername` _can_ be `null` according to the types, but the generated `selectUserById` won't accept that. A simple fix is to check if it exists and just return early if it doesn't.
 
 ### Normalizing the Notifications Slice