doc: fix typo in release notes

Change-Id: I5310cef72e714b22bcf2ae9e6fd85dbb7e8a15a2
Reviewed-on: https://go-review.googlesource.com/13787Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

doc/go1.5.html

@@ -540,7 +540,7 @@ This acts as a custom replacement for <code>go tool</code>.
 The <code>test</code> subcommand now has a <code>-count</code>
 flag to specify how many times to run each test and benchmark.
 The <a href="/pkg/testing/"><code>testing</code></a> package
-does the work here, through by the <code>-test.count</code> flag.
+does the work here, through the <code>-test.count</code> flag.
 </li>
 
 <li>