squash: doc fixup

Author: LiviaMedeiros

test/common/README.md

@@ -301,8 +301,8 @@ used as the error message for the `AssertionError`.
 
 ### `mustNotMutate(object)`
 
-* `object` [\<Object>][<Object>]
-* return [\<Object>][<Object>]
+* `object` [\<Object>][<Object>] | [\<Function>][<Function>]
+* return [\<Object>][<Object>] | [\<Function>][<Function>]
 
 Returns a proxy object that triggers an `AssertionError` on mutation attempt,
 including mutation of deeply nested objects.