Update README.md

The config header is "Gitlab Merge Request Builder", not "Gitlab Merge Requests Builder" (i.e. Request, not Requests), makes it easier to follow instructions directly and Ctrl+F on the page. If "Requests" would be better I believe this is the corresponding bit on the config page itself: https://github.com/timols/jenkins-gitlab-merge-request-builder-plugin/blob/1.2.2/src/main/resources/org/jenkinsci/plugins/gitlab/GitlabBuildTrigger/global.jelly#L2