Update osd_symbols.h

Author: breadoven

src/main/drivers/osd_symbols.h

@@ -175,7 +175,7 @@
 #define SYM_FLIGHT_HOURS_REMAINING  0xDB  // 219 Flight time (hours) remaining
 #define SYM_GROUND_COURSE           0xDC  // 220 Ground course
 
-#define SYM_TERRAIN_FOLLOWING       0xFB  // 251 General alert symbol
+#define SYM_TERRAIN_FOLLOWING       0xFB  // 251 Terrain following (also Alt adjust)
 #define SYM_CROSS_TRACK_ERROR       0xFC  // 252 Cross track error
 
 #define SYM_LOGO_START              0x101 // 257 to 297, INAV logo