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/23673Reviewed-by: Ian Lance Taylor <iant@golang.org>

doc/go1.7.html

@@ -362,7 +362,7 @@ packages.
 <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>,