The term is only used in the menu entry and the command
"Show Contextual Help". In other locations in the UI this is called
"Context Help". So we change this to "Show Context Help" to make it
consistent.

Change-Id: Iebe38dfc9433107769da42226af16f28e8504b30
Reviewed-on: https://git.eclipse.org/r/c/platform/eclipse.platform.ui/+/181602
Tested-by: Platform Bot <platform-bot@eclipse.org>
Tested-by: Lars Vogel <Lars.Vogel@vogella.com>
Reviewed-by: Lars Vogel <Lars.Vogel@vogella.com>
Reviewed-by: Matthias Becker <ma.becker@sap.com>