AP_Mount: alexmos minor spell fix

libraries/AP_Mount/AP_Mount_Alexmos.h

@@ -78,7 +78,7 @@ public:
     // update mount position - should be called periodically
     void update() override;
 
-    // has_pan_control - returns true if this mount can control it's pan (required for multicopters)
+    // has_pan_control - returns true if this mount can control its pan (required for multicopters)
     bool has_pan_control() const override;
 
     // set_mode - sets mount's mode