doc/go1.7.html: fix spelling of cancelation

We say "cancelation," not "cancellation."

Fixes #15928.

Change-Id: I66d545404665948a27281133cb9050eebf1debbb
Reviewed-on: https://go-review.googlesource.com/23673
Reviewed-by: Ian Lance Taylor <iant@golang.org>
diff --git a/doc/go1.7.html b/doc/go1.7.html
index 9ee377e..37e1e56 100644
--- a/doc/go1.7.html
+++ b/doc/go1.7.html
@@ -362,7 +362,7 @@
 <p>
 Go 1.7 moves the <code>golang.org/x/net/context</code> package
 into the standard library as <a href="/pkg/context/"><code>context</code></a>.
-This allows the use of contexts for cancellation, timeouts, and passing
+This allows the use of contexts for cancelation, timeouts, and passing
 request-scoped data in other standard library packages,
 including
 <a href="#net">net</a>,