doc/go1.13: fix typo

Change-Id: I99e76c0c12050289be5b353595eb21fbabe7c01e
Reviewed-on: https://go-review.googlesource.com/c/go/+/184597Reviewed-by: Ian Lance Taylor <iant@golang.org>

doc/go1.13.html

@@ -341,7 +341,7 @@ go env -w GOSUMDB=off
 <p id="go-env-w"><!-- CL 171137 -->
   The <a href="/cmd/go/#hdr-Environment_variables"><code>go</code> <code>env</code></a>
   command now accepts a <code>-w</code> flag to set the per-user default value
-  of an enviroment variable recognized by the
+  of an environment variable recognized by the
   <code>go</code> command, and a corresponding <code>-u</code> flag to unset a
   previously-set default. Defaults set via
   <code>go</code> <code>env</code> <code>-w</code> are stored in the