Fix writing of the article with a lowercase letter

Author: lex111

frontend.rst

@@ -51,7 +51,7 @@ Optimizing
 ..........
 
 * :doc:`Versioning (and the manifest.json file) </frontend/encore/versioning>`
-* :doc:`Using A CDN </frontend/encore/cdn>`
+* :doc:`Using a CDN </frontend/encore/cdn>`
 * :doc:`Creating a "Shared" entry for re-used modules </frontend/encore/shared-entry>`
 * :doc:`/frontend/encore/url-loader`