Fix Doxygen documentation for the Apex rotator

Fixes:
.../include/hamlib/rotlist.h:672: warning: documentation for unknown define ROT_MODEL_APEX found.
.../include/hamlib/rotlist.h:680: warning: Member ROT_MODEL_APEX_SHARED_LOOP (macro definition) of group rotator is not documented.

include/hamlib/rotlist.h

@@ -669,7 +669,7 @@
 /**
  * \brief A macro that returns the model number of the APEX backend.
  *
- * \def ROT_MODEL_APEX
+ * \def ROT_MODEL_APEX_SHARED_LOOP
  *
  * The APEX backend can be used with APEX * rotators. 
  */