minor #6120 fix README requirements link (garak)

This PR was merged into the 2.8 branch.

Discussion
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fix README requirements link

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | 2.8, 2.7, 2.3
| Fixed tickets |

The "requirements" secion is not in README anymore, so we need to point to specific version

Commits
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35c2b00 fix README requirements link

Author: xabbuh

reference/requirements.rst

@@ -56,4 +56,4 @@ If you want to use Doctrine, you will need to have PDO installed. Additionally,
 you need to have the PDO driver installed for the database server you want
 to use.
 
-.. _`Requirements section of the README`: https://github.com/symfony/symfony/blob/2.7/README.md#requirements
+.. _`Requirements section of the README`: https://github.com/symfony/symfony/blob/2.8/README.md#requirements