graphics: Fix name for stealth sphere.
graphics: Fix name for stealth sphere.

The invisibility power-up is named 'Stealth Sphere' in the Freedoom
DEHACKED lump. Change the HELP screen to match this for consistency.

CWolfRU/freedoom,jmtd/freedoom,jmtd/freedoom,CWolfRU/freedoom,jmtd/freedoom