minor #13314 [Form] tweaked a deprecation message (fabpot)

This PR was merged into the 2.7 branch.

Discussion
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[Form] tweaked a deprecation message

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | n/a
| License       | MIT
| Doc PR        | n/a

While upgrading an app to 2.7, I found this message unclear. To be honest, and like many developers, I did not read the entire message and I missed the "cast to a string" part.

Commits
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a396b41 [Form] tweaked a deprecation message

Author: fabpot

Form.php

@@ -848,7 +848,7 @@ public function getErrors($deep = false, $flatten = true)
      */
     public function getErrorsAsString($level = 0)
     {
-        trigger_error('The '.__METHOD__.' method is deprecated since version 2.5 and will be removed in 3.0. Use the Form::getErrors(true, false) method instead and cast the result to a string.', E_USER_DEPRECATED);
+        trigger_error('The '.__METHOD__.' method is deprecated since version 2.5 and will be removed in 3.0. Use (string) Form::getErrors(true, false) instead.', E_USER_DEPRECATED);
 
         return self::indent((string) $this->getErrors(true, false), $level);
     }