WSUG: Minor fixups for the statistics chapter.

Fix a quotation warning. Use the menu:[] and button:[] macros. Other
minor changes. Clean up whitespace.

Change-Id: I3ae98ddcbd90c8d22284a9ef467268dabee8f829
Reviewed-on: https://code.wireshark.org/review/3806
Reviewed-by: Gerald Combs <gerald@wireshark.org>