docs: add missed verb in docs (#15550)

Author: jmosawy

docs/rules/no-ex-assign.md

@@ -2,7 +2,7 @@
 
 Disallows reassigning exceptions in `catch` clauses.
 
-If a `catch` clause in a `try` statement accidentally (or purposely) assigns another value to the exception parameter, it impossible to refer to the error from that point on.
+If a `catch` clause in a `try` statement accidentally (or purposely) assigns another value to the exception parameter, it is impossible to refer to the error from that point on.
 Since there is no `arguments` object to offer alternative access to this data, assignment of the parameter is absolutely destructive.
 
 ## Rule Details