AP_Terrain: minor comment fix

libraries/AP_Terrain/AP_Terrain.h

@@ -153,11 +153,10 @@ public:
                                          bool extrapolate = false);
 
     /* 
-       return current height above terrain at current AHRS
-       position. 
+       return current height above terrain at current AHRS position.
 
        If extrapolate is true then extrapolate from most recently
-       available terrain data is terrain data is not available for the
+       available terrain data if terrain data is not available for the
        current location.
 
        Return true if height is available, otherwise false.