net: change "mask" to "prefix length" in ParseCIDR comment

Fixes #18175

Change-Id: I0074109fa885976b22c2be9fa39641d4d0657dee
Reviewed-on: https://go-review.googlesource.com/34375Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/net/ip.go

@@ -654,7 +654,7 @@ func ParseIP(s string) IP {
 	return nil
 }
 
-// ParseCIDR parses s as a CIDR notation IP address and mask,
+// ParseCIDR parses s as a CIDR notation IP address and prefix length,
 // like "192.0.2.0/24" or "2001:db8::/32", as defined in
 // RFC 4632 and RFC 4291.
 //