add type hint in documentation

Author: syl20b

docs/en/dbal-type.rst

@@ -24,12 +24,12 @@ To register a custom DBAL type, create a class that extends
             return $platform->getDecimalTypeDeclarationSQL($column);
         }
 
-        public function convertToPHPValue($value, AbstractPlatform $platform): mixed
+        public function convertToPHPValue(mixed $value, AbstractPlatform $platform): mixed
         {
             return $value;
         }
 
-        public function convertToDatabaseValue($value, AbstractPlatform $platform): mixed
+        public function convertToDatabaseValue(mixed $value, AbstractPlatform $platform): mixed
         {
             return $value;
         }