Doc: use less-awkward phrasing.

Improve wording in note about tools required to build
from the source repository.

Laurenz Albe, per gripe from Riivo Kolka

Discussion: https://postgr.es/m/167463493588.2667301.13267758265445155872@wrigleys.postgresql.org

doc/src/sgml/sourcerepo.sgml

@@ -19,9 +19,10 @@
   <para>
    Note that building <productname>PostgreSQL</productname> from the source
    repository requires reasonably up-to-date versions of <application>bison</application>,
-   <application>flex</application>, and <application>Perl</application>. These tools are not needed
-   to build from a distribution tarball, because the files that these tools
-   are used to build are included in the tarball.  Other tool requirements
+   <application>flex</application>, and <application>Perl</application>.
+   These tools are not needed to build from a distribution tarball, because
+   the files generated with these tools are included in the tarball.
+   Other tool requirements
    are the same as shown in <xref linkend="install-requirements"/>.
   </para>