go.talks/pkg/present: remove tilde from bad url chars

Obviously we should be able to use tilde in URLs.

R=golang-dev, minux.ma, r, noah.evans
CC=golang-dev
https://golang.org/cl/7443052
diff --git a/pkg/present/link.go b/pkg/present/link.go
index d683f02..c9fd449 100644
--- a/pkg/present/link.go
+++ b/pkg/present/link.go
@@ -61,7 +61,7 @@
 	}
 	urlEnd := strings.Index(s, "]")
 	url := s[2:urlEnd]
-	const badURLChars = `<>"{}|\^~[] ` + "`" // per RFC1738 section 2.2
+	const badURLChars = `<>"{}|\^[] ` + "`" // per RFC2396 section 2.4.3
 	if strings.ContainsAny(url, badURLChars) {
 		return
 	}