Update fern/docs/pages/customization.mdx

Co-authored-by: Atul-Butola <atul.butola@devrev.ai>

Author: shivansh

fern/docs/pages/customization.mdx

@@ -657,5 +657,5 @@ stage display name is not used in the expression because it is liable to change.
 </Callout>
 
 Constraints on stock fields can only be strengthened.
-For example, a required stock field cannot be made optional and a hidden stock
+For example, a required stock field cannot be made optional, and a hidden stock
 field cannot be made visible.