update 'mergeGithubPullRequest' to 'mergePullRequest' in Job DSL section of README
update 'mergeGithubPullRequest' to 'mergePullRequest' in Job DSL section of README

DavidTanner/ghprb-plugin,dulvac/ghprb-plugin,arcadia/ghprb-plugin,fbelzunc/ghprb-plugin,jenkinsci/ghprb-plugin,dulvac/ghprb-plugin,fbelzunc/ghprb-plugin,arcadia/ghprb-plugin,jenkinsci/ghprb-plugin,DavidTanner/ghprb-plugin