fix REALLY BAD TYPO in GradeDisplay:SetGrade Lua function.

this is dumb and I'm too lazy to find out who's REALLY to blame so I want to assume it's my fault, probably. If anyone wants to bother clearing my name, check the stepmania SVN history. I don't care enough :v

src/GradeDisplay.cpp

@@ -47,7 +47,7 @@ public:
 	}
 	static int SetGrade( T* p, lua_State *L )
 	{
-		Grade g = Enum::Check<Grade>(L, 2);
+		Grade g = Enum::Check<Grade>(L, 1);
 		p->SetGrade( g );
 		return 0;
 	}