Rework installation instructions again

Dept. of second thoughts: the new verbiage added in commit aaec237b1a
is targeted at the wrong audience.  Remove the bits about git and talk
about how to get tarballs only.  People looking for the git repo can
look in the appendix.  That'll need to be expanded, but this commit
doesn't do that.

In passing, fix a couple of typos that snuck in with the previous
commit.

Discussion: https://postgr.es/m/713760.1626891263@sss.pgh.pa.us

doc/src/sgml/installation.sgml

@@ -354,29 +354,13 @@ su - postgres
   <title>Getting the Source</title>
 
   <para>
-   The version control repository for <productname>PostgreSQL</productname>
-   can be obtained from
-   <ulink url="https://git.postgresql.org/gitweb/?p=postgresql.git;a=summary"></ulink>;
-   this is the recommended way to obtain the
-   <productname>PostgreSQL</productname> source code.
-   You can obtain a clone of the repository by running
-<screen>
-<userinput>git clone https://git.postgresql.org/git/postgresql.git</userinput>
-</screen>
-   This will create a directory <filename>postgresql</filename> under the current
-   directory with the <productname>PostgreSQL</productname> sources.
-   Change into that directory for the rest of the installation procedure.
-   See <xref linkend="git"/> for further information.
-  </para>
-
-  <para>
-   Alternatively, source code for the released versions can be obtained
-   from the download section of our website:
+   The <productname>PostgreSQL</productname> source code for released versions
+   can be obtained from the download section of our website:
    <ulink url="https://www.postgresql.org/ftp/source/"></ulink>.
-   Download then
+   Download the
    <filename>postgresql-<replaceable>version</replaceable>.tar.gz</filename>
    or <filename>postgresql-<replaceable>version</replaceable>.tar.bz2</filename>
-   you're interested in, then unpack it:
+   file you're interested in, then unpack it:
 <screen>
 <userinput>tar xf postgresql-<replaceable>version</replaceable>.tar.bz2</userinput>
 </screen>
@@ -385,6 +369,11 @@ su - postgres
    the current directory with the <productname>PostgreSQL</productname> sources.
    Change into that directory for the rest of the installation procedure.
   </para>
+
+  <para>
+   Alternatively, you can use the Git version control system; see
+   <xref linkend="git"/> for more information.
+  </para>
  </sect1>
 
  <sect1 id="install-procedure">