Small possible clarification in pg_basebackup reference page

The <literal> markup is not visible as distinct on man pages, which
creates a bit of confusion when looking at the documentation of the
pg_basebackup -l option.  Rather than reinventing the entire font
system for man pages to remedy this, just put some quotes around this
particular case, which should also help in other output formats.

doc/src/sgml/ref/pg_basebackup.sgml

@@ -284,7 +284,7 @@ PostgreSQL documentation
       <listitem>
        <para>
         Sets the label for the backup. If none is specified, a default value of
-        <literal>pg_basebackup base backup</literal> will be used.
+        <quote><literal>pg_basebackup base backup</literal></quote> will be used.
        </para>
       </listitem>
      </varlistentry>