core: Fix wrong return value docs

ostree_commit_get_parent() returns a string form of a checksum, not a
binary form.

Closes: #360
Approved by: cgwalters

src/libostree/ostree-core.c

@@ -1994,7 +1994,8 @@ ostree_validate_structureof_dirmeta (GVariant      *dirmeta,
  * ostree_commit_get_parent:
  * @commit_variant: Variant of type %OSTREE_OBJECT_TYPE_COMMIT
  *
- * Returns: Binary checksum with parent of @commit_variant, or %NULL if none
+ * Returns: Checksum of the parent commit of @commit_variant, or %NULL
+ * if none
  */
 gchar *
 ostree_commit_get_parent (GVariant  *commit_variant)