fix gsm_7bit_decode API comment: septets, not octets
diff --git a/include/osmocom/gsm/gsm_utils.h b/include/osmocom/gsm/gsm_utils.h
index 6431866..6458447 100644
--- a/include/osmocom/gsm/gsm_utils.h
+++ b/include/osmocom/gsm/gsm_utils.h
@@ -66,7 +66,8 @@
* Requires n >= 1.
* \param user_data A pointer to the start of the packed 7bit character
* sequence.
- * \param length The length of the input sequence (in octets).
+ * \param length The length of the input sequence in septets, for
+ * example pass octet_length*8/7.
*
* \returns the number of (8 bit) chars written excluding the terminating \0.
* This is the same like strlen(decoded).