CLEANUP: standard: Fix typo in IPv6 mask example

IPv6 addresses with two double colons are invalid.

This typo was introduced in commit 471851713af20d84b67b8966471ea758dc8c12b9.
diff --git a/src/standard.c b/src/standard.c
index b6bb1bc..d87e09c 100644
--- a/src/standard.c
+++ b/src/standard.c
@@ -1037,7 +1037,7 @@
 }
 
 /* converts <str> to a struct in6_addr containing a network mask. It can be
- * passed in quadruplet form (ffff::ffff::) or in CIDR form (64). It returns 1
+ * passed in quadruplet form (ffff:ffff::) or in CIDR form (64). It returns 1
  * if the conversion succeeds otherwise zero.
  */
 int str2mask6(const char *str, struct in6_addr *mask)