docs(readme): fix broken TYPO3 Legacy Management link

The link to "Managing Extensions - Legacy Guide" in the Extension
Manager section was pointing at the 'main' (v13) branch of the TYPO3
getting-started tutorial, where `Extensions/LegacyManagement.html`
returns 404 (the legacy-management page was removed from main). The
page is still published under v11.5 (the last LTS that ships the
legacy extension manager), so pin the link to that version.

Closes #179

Author: MukundaKatta

README.md

@@ -18,7 +18,7 @@ composer require ichhabrecht/content-defender
 
 ### Extension Manager
 
-See [TYPO3 Documentation - Managing Extensions - Legacy Guide](https://docs.typo3.org/m/typo3/tutorial-getting-started/main/en-us/Extensions/LegacyManagement.html)
+See [TYPO3 Documentation - Managing Extensions - Legacy Guide](https://docs.typo3.org/m/typo3/tutorial-getting-started/11.5/en-us/Extensions/LegacyManagement.html#extensions_legacy_management)
 
 ## Usage