Missing equals sign in agent_windows

According to documentation: https://github.com/gocd/gocd/blob/94237eb8d1136a89ee27fb401440adaa39d5b7e9/helper/topics/installing_go_agent.xml
Merge pull request #20 from naiduv/patch-1

Missing equals sign in agent_windows

I can't test this myself, but please shout if it introduces any problems.