doc: two minor edits to contribute.html

- Missing parenthesis
- Fix bad phrasing

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/7063048

doc/contribute.html

@@ -121,7 +121,7 @@ will show documentation for its commands.
 The code review server uses a Google Account to authenticate.
 (If you can use the account to
 <a href="https://www.google.com/accounts/Login?hl=en&amp;continue=http://www.google.com/">sign in at google.com</a>,
-you can use it to sign in to the code review server.
+you can use it to sign in to the code review server.)
 The email address you use on the Code Review site
 will be recorded in the <a href="http://code.google.com/p/go/source/list">Mercurial change log</a>
 and in the <a href="/CONTRIBUTORS"><code>CONTRIBUTORS</code></a> file.
@@ -502,7 +502,7 @@ and perhaps the <a href="/AUTHORS"><code>AUTHORS</code></a> file.
 
 <p>The <a href="/CONTRIBUTORS"><code>CONTRIBUTORS</code></a> file
 defines who the Go contributors&mdash;the people&mdash;are;
-the <a href="/AUTHORS"><code>AUTHORS</code></a> file, which defines
+the <a href="/AUTHORS"><code>AUTHORS</code></a> file defines
 who &ldquo;The Go Authors&rdquo;&mdash;the copyright holders&mdash;are.
 The Go developers at Google will update these files when submitting
 your first change.