[lldb/Commands] Align process launch --plugin with process attach (NFC)

Following `7169d3a3`, this patch updates
the short option for the plugin command option to (`-p` to `-P`) to
align with the `process attach` command options.

The long option remains the same since there are already the same for both
commands.

Signed-off-by: Med Ismail Bennani <medismail.bennani@gmail.com>