fix typo according to docs and tests

lib/api/projects.rb

@@ -37,11 +37,11 @@ module Gitlab
       #
       # Parameters:
       #   id (required) - The ID or code name of a project
-      #   branch_id (required) - The name of the branch
+      #   branch (required) - The name of the branch
       # Example Request:
-      #   GET /projects/:id/repository/branches/:branch_id
-      get ":id/repository/branches/:branch_id" do
-        @branch = user_project.repo.heads.find { |item| item.name == params[:branch_id] }
+      #   GET /projects/:id/repository/branches/:branch
+      get ":id/repository/branches/:branch" do
+        @branch = user_project.repo.heads.find { |item| item.name == params[:branch] }
         present @branch, :with => Entities::RepoObject
       end