Small possible clarification in pg_basebackup reference page

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.