Fix doxygen error in mbed_retarget.cpp

Add add documentation to the second parameter of handle_open_errors.
This is in preparation for turning on doxygen for source files.

Author: c1728p9

platform/mbed_retarget.cpp

@@ -131,6 +131,8 @@ static void init_serial() {
  *
  * @param error is a negative error code returned from an mbed function and
  *              will be negated to store a positive error code in errno
+ * @param filehandle_idx is the index of the filehandle to remove
+ *
  */
 static int handle_open_errors(int error, unsigned filehandle_idx) {
     errno = -error;