aper: Fix documentation of aper_put_length()

Fixed as seen the function is being called in most placed where
need_eom is true.

skeletons/aper_support.h

@@ -33,7 +33,8 @@ ssize_t aper_get_nsnnwn(asn_per_data_t *pd, int range);
  * X.691 (08/2015) #11.9 "General rules for encoding a length determinant"
  * Put the length "whole_length" to the Aligned PER stream.
  * If (opt_need_eom) is given, it will be set to 1 if final 0-length is needed.
- * In that case, invoke uper_put_length(po, 0, 0) after encoding the last block.
+ * In that case, invoke aper_put_length(po, -1, 0, NULL) after encoding the last
+ * block.
  * This function returns the number of units which may be flushed
  * in the next units saving iteration.
  */