Fix 'The symbol which represents that a class implements another class is the...

Fix 'The symbol which represents that a class implements another class is the same as the symbol used for inheritance between classes'.

The main bug has been fixed in 2.9.5; this commit completes by
fixing the misleading term 'Implements' for the 'Generalization' arrow
tooltip.

CCBUG:311808
FIXED-IN:2.20.80 (KDE Applications 16.11.80)