Fix function documentation to correctly mirror code

Currently the function documentation states that two strings are 
allocated, this is outdated. Only one char ** parameter is passed 
into the function now, clearly only a pointer to a single string 
is returned and needs to be free'd.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Tobin C. Harding <me@tobin.cc>
Closes #6754

lib/libzfs/libzfs_import.c

@@ -2296,7 +2296,7 @@ find_aux(zpool_handle_t *zhp, void *data)
 
 /*
  * Determines if the pool is in use.  If so, it returns true and the state of
- * the pool as well as the name of the pool.  Both strings are allocated and
+ * the pool as well as the name of the pool.  Name string is allocated and
  * must be freed by the caller.
  */
 int