Fix markdown syntax in UPGRADE.md

Author: sstok

UPGRADE.md

@@ -6,7 +6,7 @@ From 3.x to 4.0
 
 * Support for Symfony 6 was removed, PHP 8.4 and Symfony 7.4 is now the minimum required version.
 
-* The constructor of password constraints has changed, see (Upgrade instructions of the validator library)[https://github.com/rollerworks/PasswordStrengthValidator/blob/main/UPGRADE.md]
+* The constructor of password constraints has changed, see [Upgrade instructions of the validator library][https://github.com/rollerworks/PasswordStrengthValidator/blob/main/UPGRADE.md]
 
 From 2.x to 3.0
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