Fix lint warning by renaming MaxBytesExceededErr -> ErrMaxBytesExceeded (#1676)

Signed-off-by: Jonas Östanbäck <jonas.ostanback@gmail.com>

caddyhttp/httpserver/replacer.go

@@ -302,7 +302,7 @@ func (r *replacer) getSubstitution(key string) string {
 		}
 		_, err := ioutil.ReadAll(r.request.Body)
 		if err != nil {
-			if err == MaxBytesExceededErr {
+			if err == ErrMaxBytesExceeded {
 				return r.emptyValue
 			}
 		}