correct small typos in comments

ArduPlane/config.h

@@ -519,7 +519,7 @@
 
 
 
-/*  The following parmaeters have no corresponding control implementation
+/*  The following parameters have no corresponding control implementation
 #ifndef THROTTLE_ALT_P
 # define THROTTLE_ALT_P         0.32
 #endif

libraries/AP_Common/AP_Param.h

@@ -22,7 +22,7 @@
 #define AP_MAX_NAME_SIZE 15
 #define AP_NESTED_GROUPS_ENABLED
 
-// a varient of offsetof() to work around C++ restrictions.
+// a variant of offsetof() to work around C++ restrictions.
 // this can only be used when the offset of a variable in a object
 // is constant and known at compile time
 #define AP_VAROFFSET(type, element) (((uintptr_t)(&((const type *)1)->element))-1)