Documentation/trace/uprobetracer.txt: fix incorrect examples

Current uprobetracer examples don't work as they use an incorrect syntax
- if no event name is specified then 'p/r' must not be followed by a
  colon
- if no event name is specified then the default event name will have a
  'p_' prefix, so use that in the '-' example as well

Signed-off-by: Marcin Nowakowski <marcin.nowakowski@imgtec.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>