Doc: clarify wording about tool requirements in sourcerepo.sgml.

Original wording had confusingly vague antecedent for "they", so replace
that with a more repetitive but clearer formulation.  In passing, make the
link to the installation requirements section more specific.  Per gripe
from Martin Mai, though this is not the fix he initially proposed.

Discussion: https://postgr.es/m/CAN_NWRu-cWuNaiXUjV3m4H-riWURuPW=j21bSaLADs6rjjzXgQ@mail.gmail.com

doc/src/sgml/sourcerepo.sgml

@@ -20,9 +20,9 @@
    Note that building <productname>PostgreSQL</productname> from the source
    repository requires reasonably up-to-date versions of <application>bison</>,
    <application>flex</>, and <application>Perl</>. These tools are not needed
-   to build from a distribution tarball since the files they are used to build
-   are included in the tarball.  Other tool requirements are the same as shown
-   in <xref linkend="installation">.
+   to build from a distribution tarball, because the files that these tools
+   are used to build are included in the tarball.  Other tool requirements
+   are the same as shown in <xref linkend="install-requirements">.
   </para>
 
  <sect1 id="git">