Add missing _ for external link reference

doc/how-to-source-packages.rst

@@ -27,7 +27,7 @@ This is described in the :ref:`package consumer<pkg-consumers>` section.
    description". That's the ``.cabal`` file. The other will be the "Cabal source
    package". That's the ``.tar.gz`` archive containing all of the source for the
    package.  For example, here's the direct link to download
-   `vector-0.13.1.0.tar.gz <vector-download>`_.
+   `vector-0.13.1.0.tar.gz <vector-download_>`_.
 
    Conversely, package *source code* may be only available from a source code
    repository.  This is the case for all unpublished packages and for all
@@ -140,7 +140,7 @@ picked up from the source code repository instead of from Hackage.
    they share many of the same :ref:`VCS fields<vcs-fields>`. Let's see an
    example, again using the ``vector`` package;
 
-   Looking at vector's `package description <vector-pkg-desc>`_ on Hackage we
+   Looking at vector's `package description <vector-pkg-desc_>`_ on Hackage we
    see the ``source-repository`` stanza:
 
    .. code-block:: cabal