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. If "Requests" would be better I believe this bit should be changed instead: 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