doc/contribute: directly link to the Agreements page

There's no Settings->Agreement path for PolyGerrit users, but if we
link directly to the page in the instructions, Gerrit will inform them
that they can access the page by switching to the old UI.

Fixes #20207

Change-Id: I0887ee854e4ac5975b5f305adb6259b81b41618f
Reviewed-on: https://go-review.googlesource.com/42412Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

doc/contribute.html

@@ -118,10 +118,10 @@ it does not need to be completed again.</i>
 You can see your currently signed agreements and sign new ones through the Gerrit
 interface.
 To do this, <a href="https://go-review.googlesource.com/login/">Log into Gerrit</a>,
-click your name in the upper-right, choose "Settings", then select "Agreements"
-from the topics on the left.
-If you do not have a signed agreement listed here,
-you can create one by clicking "New Contributor Agreement" and following the steps.
+then visit the <a href="https://go-review.googlesource.com/settings/agreements">Agreements</a>
+page.
+If you do not have a signed agreement listed there, you can create one
+by clicking "New Contributor Agreement" and following the steps.
 </p>
 
 <p>