Minor updates to meterpreter persistence module.

Slight cleanup (e.g. to keep lines less than 100 characters).
Clarification within the help text.
Resolve a minor bug in the cleanup resource file created by this module to escape the windows path separator and to not attempt to kill the agent process (this would not have worked as the pid is likely to have changed, e.g. as a result of a target system reboot).

NB: This is my first contribution so is a deliberately minor change so as to familiarise myself with the process.