doc/go1.9: fix typo in Moved GOROOT

Change-Id: I71bfff6a3462e6dfd7a65ef76ec56644bae37c34
Reviewed-on: https://go-review.googlesource.com/57272Reviewed-by: Avelino <t@avelino.xxx>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

doc/go1.9.html

@@ -163,7 +163,7 @@ type T1 = T2
   was invoked to attempt to locate the root of the Go install tree.
   This means that if the entire Go installation is moved to a new
   location, the go tool should continue to work as usual.
-  This may be overriden by setting <code>GOROOT</code> in the environment,
+  This may be overridden by setting <code>GOROOT</code> in the environment,
   which should only be done in unusual circumstances.
   Note that this does not affect the result of
   the <a href="/pkg/runtime/#GOROOT">runtime.GOROOT</a> function, which