doc: fix typo in Go 1.9 release notes

Change-Id: Ia5c8fcff000bf3d369d42407342c470ea92e18da
Reviewed-on: https://go-review.googlesource.com/46006Reviewed-by: Alberto Donizetti <alb.donizetti@gmail.com>

doc/go1.9.html

@@ -259,7 +259,7 @@ CL 40331: https://golang.org/cl/40331: cmd/link,runtime/cgo: enable PT_TLS gener
 <p> <!-- CL 38796 -->
   The
   new <a href="/pkg/testing/#T.Helper"><code>(*T).Helper</code></a>
-  an <a href="/pkg/testing/#B.Helper"><code>(*B).Helper</code></a>
+  and <a href="/pkg/testing/#B.Helper"><code>(*B).Helper</code></a>
   methods mark the calling function as a test helper function.  When
   printing file and line information, that function will be skipped.
   This permits writing test helper functions while still having useful