Update capitalization in runner docs

What does this MR do?

Updates the capitalization in runner docs, but either adding the correct capitalization, or adding backticks when needed.

Used this array for markdownlint:

  "proper-names": {
    "names": [
      "Git",
      "GitLab",
      "git-annex",
      "Git LFS",
      "JavaScript",
      "Jira",
      "Jenkins",
      "GitLab Runner",
      "Kubernetes",
      "Debian",
      "Ubuntu",
      "GitLab Shell",
      "Omnibus GitLab",
      "PostgreSQL",
      "Nginx",
      "Nginx Ingress",
      "Unicorn",
      "GitLab Workhorse",
      "Redis",
      "Gitaly",
      "Python",
      "runit"
    ],
    "code_blocks": false
  }

Related to https://gitlab.com/gitlab-org/gitlab-ce/issues/66484