docs: fix by proofreading

Co-authored-by: Abdul Malik Ikhsan <[email protected]>

Author: kenjis

user_guide_src/source/models/model.rst

@@ -141,7 +141,7 @@ This array should be updated with the field names that can be set during ``save(
 against just taking input from a form and throwing it all at the model, resulting in
 potential mass assignment vulnerabilities.
 
-.. note:: The ``$primaryKey`` should never be an allowed field.
+.. note:: The ``$primaryKey`` field should never be an allowed field.
 
 $useTimestamps
 --------------