minor #6399 Fixed wrong code examples for Isbn constraint (AAstakhov)

This PR was submitted for the 2.5 branch but it was merged into the 2.7 branch instead (closes #6399).

Discussion
----------

Fixed wrong code examples for Isbn constraint

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | 2.5+
| Fixed tickets | n/a

The example in the annotations code block causes an exception '[Semantical Error] Couldn't find constant isbn10'.
It happens because attribute values in annotation blocks must be quoted.
Additionally I fixed PHP code block to use quoted attribute also there

Commits
-------

503c07c Fixed wrong code examples for Isbn constraint

reference/constraints/Isbn.rst

@@ -48,8 +48,8 @@ on an object that will contain an ISBN.
         {
             /**
              * @Assert\Isbn(
-             *     type = isbn10,
-             *     message: This value is not  valid.
+             *     type = "isbn10",
+             *     message = "This value is not  valid."
              * )
              */
             protected $isbn;
@@ -99,7 +99,7 @@ on an object that will contain an ISBN.
             public static function loadValidatorMetadata(ClassMetadata $metadata)
             {
                 $metadata->addPropertyConstraint('isbn', new Assert\Isbn(array(
-                    'type'    => isbn10,
+                    'type'    => 'isbn10',
                     'message' => 'This value is not valid.'
                 )));
             }