Fix link in docs.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@308046 91177308-0d34-0410-b5e6-96231b3b80d8

include/clang/Basic/AttrDocs.td

@@ -1027,7 +1027,7 @@ Starting with the macOS 10.12 SDK, the ``API_AVAILABLE`` macro from
   @end
 
 Also see the documentation for `@available
-<http://clang.llvm.org/docs/LanguageExtensions.html#objective-c-@available>`_
+<http://clang.llvm.org/docs/LanguageExtensions.html#objective-c-available>`_
   }];
 }