Gtk Utils: fix unknown command tag name 'parm'; did you mean 'param'? (-Wdocumentation)

Change-Id: I89f242bf916d22e0a4a665696ec3fe5ee02b8a1f
Reviewed-on: https://code.wireshark.org/review/5283
Reviewed-by: Anders Broman <a.broman58@gmail.com>