ProcessDialog: Remove stray 'this.$' from documentation code example
ProcessDialog: Remove stray 'this.$' from documentation code example

Follows-up c38a542.

Change-Id: Ie377265e906aaebda99af51bf7095dc28e03439c

wikimedia/oojs-ui,wikimedia/oojs-ui,wikimedia/oojs-ui,wikimedia/oojs-ui