vlc_modules: fix doxygen warnings

include/vlc_modules.h:77: warning: argument 'capability' of command @param is not found in the argument list of vlc_module_load(struct vlc_logger *log, const char *cap, const char *name, bool strict, vlc_activate_t probe,...)

    include/vlc_modules.h:77: warning: The following parameter of vlc_module_load(struct vlc_logger *log, const char *cap, const char *name, bool strict, vlc_activate_t probe,...) is not documented:
      parameter 'cap'

include/vlc_modules.h

@@ -84,7 +84,7 @@ void *vlc_module_map(struct vlc_logger *log, module_t *mod);
  * support arbitrary prototypes for the module entry point.
  *
  * \param log logger for debugging (or NULL to ignore)
- * \param capability capability, i.e. class of module
+ * \param cap capability, i.e. class of module
  * \param name name of the module asked, if any
  * \param strict if true, do not fallback to plugin with a different name
  *                 but the same capability