doc: make path platform dependent

Path should now appear with the correct slash, depending on which
platform install document is being viewed - keeping in line with the
rest of the document.

Fixes #30160

Change-Id: Ib10e5a4adf366c700bff6f8d246bd5e3111ed61c
Reviewed-on: https://go-review.googlesource.com/c/go/+/162918Reviewed-by: Andrew Bonventre <andybons@golang.org>

doc/install.html

@@ -204,7 +204,7 @@ you will need to <a href="https://golang.org/wiki/SettingGOPATH">set the <code>G
 </p>
 
 <p>
-Next, make the directory <code>src/hello</code> inside your workspace,
+Next, make the directory <code class="testUnix">src/hello</code><code class="testWindows">src\hello</code> inside your workspace,
 and in that directory create a file named <code>hello.go</code> that looks like:
 </p>