Copter: terrain spelling fix

ArduCopter/mode_guided.cpp

@@ -154,7 +154,7 @@ void ModeGuided::pva_control_start()
     // initialise yaw
     auto_yaw.set_mode_to_default(false);
 
-    // initialise terain alt
+    // initialise terrain alt
     guided_pos_terrain_alt = false;
 }
 

libraries/AC_WPNav/AC_WPNav.h

@@ -267,7 +267,7 @@ protected:
     Vector3f    _destination;           // target destination in cm from ekf origin
     float       _track_scalar_dt;       // time compression multiplier to slow the progress along the track
     float       _terrain_vel;            // maximum horizontal velocity used to ensure the aircraft can maintain height above terrain
-    float       _terrain_accel;          // acceleration value used to change _terain_vel
+    float       _terrain_accel;          // acceleration value used to change _terrain_vel
 
     // terrain following variables
     bool        _terrain_alt;   // true if origin and destination.z are alt-above-terrain, false if alt-above-ekf-origin