doc/go1.7.html: fix name of IsExist

For better or for worse, it's IsExist, not IsExists.

Change-Id: I4503f961486edd459c0c81cf3f32047dff7703a4
Reviewed-on: https://go-review.googlesource.com/24819
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

doc/go1.7.html

@@ -1062,7 +1062,7 @@ from URLs with empty query strings (like <code>/search?</code>).
 
 <dd>
 <p>
-<a href="/pkg/os/#IsExists"><code>IsExists</code></a> now returns true for <code>syscall.ENOTEMPTY</code>,
+<a href="/pkg/os/#IsExist"><code>IsExist</code></a> now returns true for <code>syscall.ENOTEMPTY</code>,
 on systems where that error exists.
 </p>