Update errors.mdx (#1151)

Improving reference to 4xxx error codes.

Author: fhightower

site/docs/messaging/errors.mdx

@@ -251,7 +251,7 @@ Bandwidth’s error code schema for messaging V2 is comprised of a 4 digit code.
 
 ### Bandwidth Detected Client Errors
 
-A 4xx code indicates that Bandwidth or the downstream carrier has identified some element of the message request unacceptable.  Repeating the request will produce the same result.
+A 4xxx code indicates that Bandwidth or the downstream carrier has identified some element of the message request unacceptable.  Repeating the request will produce the same result.
 
 | Code | Description | Friendly Description | Explanation Of Error | Billable |
 |:--|:--|:--|:--|:--|