[docs] Fix overly specific link to uploading patches on Phabricator

[ldionne]

Summary

The documentation for contributing to LLVM currently links to the section
explaining how to submit a Phabricator review using the web interface.
I believe it would be better to link to the general page for using
Phabricator instead, which explains how to sign up with Phabricator,
and also how to submit patches using either the web interface or the
command-line.

I think this is worth changing because what currently *appears* to be our
preferred way of submitting a patch (through the web interface) isn't
actually what we prefer. Indeed, patches submitted from the command-line
have more meta-data available (such as which repository the patch targets),
and also can't suffer from missing context.

[fhahn]

That seems indeed better, thanks! LGTM

[ldionne]

Fix broken link (I generated the documentation to test, this time).

llvm/docs/Contributing.rst

.. note::
   If you don't already have ``clang-format`` or ``git clang-format`` installed
   on your system, the ``clang-format`` binary will be built alongside clang, and
   the git integration can be run from
   ``clang/tools/clang-format/git-clang-format``.
 
 
 To get a patch accepted, it has to be reviewed by the LLVM community. This can
 be done using `LLVM's Phabricator`_ or the llvm-commits mailing list.
-Please  follow :ref:`Phabricator#requesting-a-review-via-the-web-interface <phabricator-request-review-web>`
+Please  follow :ref:`Phabricator#phabricator-reviews <phabricator-reviews>`
 to request a review using Phabricator.
 
 To make sure the right people see your patch, please select suitable reviewers
 and add them to your patch when requesting a review. Suitable reviewers are the
 code owner (see CODE_OWNERS.txt) and other people doing work in the area your
 patch touches. If you are using Phabricator, add them to the `Reviewers` field
 when creating a review and if you are using `llvm-commits`, add them to the CC of
 your email.