vlc_spawn: fix doxygen warnings

include/vlc_spawn.h:36: warning: argument 'path' of command @param is not found in the argument list of vlc_spawn(pid_t *pid, const char *file, const int *fdv, const char *const *argv)

    include/vlc_spawn.h:36: warning: The following parameter of vlc_spawn(pid_t *pid, const char *file, const int *fdv, const char *const *argv) is not documented:
      parameter 'file'

include/vlc_spawn.h

@@ -38,7 +38,7 @@
  * file name must be absolute.
  *
  * \param pid storage space for the child process identifier [OUT]
- * \param path executable file path [IN]
+ * \param file executable file path [IN]
  * \param fdv file descriptor array [IN]
  * \param argv NULL-terminated array of command line arguments [IN]
  *