Fix incorrect units in TabLayoutPanel example (was PX should be
EM).

Gerrit review: http://gwt-code-reviews.appspot.com/329802/show
Issue: http://code.google.com/p/google-web-toolkit/issues/detail?id=4851

Review by: jgw@google.com

git-svn-id: https://google-web-toolkit.googlecode.com/svn/trunk@8039 8db76d5a-ed1c-0410-87a9-c151d255dfc7