Fix typo in twitter link (#11529)

Summary:
A user find and reported this in issue https://github.com/facebook/rocksdb/issues/11528

Pull Request resolved: https://github.com/facebook/rocksdb/pull/11529

Reviewed By: hx235

Differential Revision: D46653618

Pulled By: jowlyzhang

fbshipit-source-id: aa98f59028daaaa5b0bd3b96ee87dcdd7369a8b9

docs/_includes/footer.html

@@ -13,7 +13,7 @@
       </div>
       <div class="footerSection">
         <a class="footerLink" href="https://github.com/facebook/rocksdb" target="_blank">GitHub</a>
-        <a class="footerLink" href="hhttps://twitter.com/rocksdb" target="_blank">Twitter</a>
+        <a class="footerLink" href="https://twitter.com/rocksdb" target="_blank">Twitter</a>
         <a class="footerLink" href="https://opensource.fb.com/legal/terms" target="_blank">Terms of Use</a>
         <a class="footerLink" href="https://opensource.fb.com/legal/privacy" target="_blank">Privacy Policy</a>
       </div>