Fix document: 'stage' is a string instead of array

Zhiyuan Lurequested to merge
gitlab-jh/jh-team/gitlab:fix-doc-about-ci-stage into master

What does this MR do and why?

What

  1. Correcting the wrong description in the documentation: Stage is an array to stage is a String
  2. Correcting the wrong example about stage in the docs

Why

Only string is valid stage, which can be easily verified.

cc @prajnamas

Edited by Zhiyuan Lu

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