design: fix URL in monotonic proposal

Change-Id: I8abe8c58e42969899601ad1acf35367f74f8b9b5
Reviewed-on: https://go-review.googlesource.com/35872
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>
diff --git a/design/12914-monotonic.md b/design/12914-monotonic.md
index 8b5a5d7..048d2b1 100644
--- a/design/12914-monotonic.md
+++ b/design/12914-monotonic.md
@@ -4,7 +4,7 @@
 
 Last updated: January 26, 2017<br>
 Discussion: [https://golang.org/issue/12914](https://golang.org/issue/12914).<br>
-URL: https://golang.org/design/12914-montonic
+URL: https://golang.org/design/12914-monotonic
 
 ## Abstract