Fix grammatical errors in URL exception message

Although the "U" at the first of URL is a vowel, the abbreviation is
pronounced "you-are-ell". Because the letter "Y" is a consonant in this
case, we should use "a" instead of "an".

Link: https://itknowledgeexchange.techtarget.com/writing-for-business/which-is-correct-a-url-or-an-url/

Signed-off-by: Henrique Moody <henriquemoody@gmail.com>

library/Exceptions/UrlException.php

@@ -15,10 +15,10 @@ class UrlException extends ValidationException
 {
     public static $defaultTemplates = [
         self::MODE_DEFAULT => [
-            self::STANDARD => '{{name}} must be an URL',
+            self::STANDARD => '{{name}} must be a URL',
         ],
         self::MODE_NEGATIVE => [
-            self::STANDARD => '{{name}} must not be an URL',
+            self::STANDARD => '{{name}} must not be a URL',
         ],
     ];
 }