fix regex-typo in lxc-monitor.sgml.in

fix regex-typo in lxc-monitor.sgml.in

To match names beginning with the letters "f" or "b" one can use
the regular expression "[fb].*" or "(f|b).*", but not "[f|b].*",
which would match strings beginning with "f", "|", or "b".

Signed-off-by: Christian von Roques <roques@z12.ch>