Correct spelling mistakes.

code/renderergl2/tr_local.h

@@ -1392,7 +1392,7 @@ typedef struct {
 	int		msec;			// total msec for backend run
 } backEndCounters_t;
 
-// all state modified by the back end is seperated
+// all state modified by the back end is separated
 // from the front end state
 typedef struct {
 	trRefdef_t	refdef;