Fix escaped angle bracket's in integration documentation.

There are several escaped angle brackets in our integration docs. While these render fine within GitLab, they are broken when rendered on doc.gitlab.com because pandoc does not escape them correctly.

You can see the problem here: http://doc.gitlab.com/ce/integration/github.html

Note that the strings <Organization> and <Your Name> are being interpreted as html tags.

It looks like doc.gitlab.com is using pandoc: https://gitlab.com/gitlab-com/doc-gitlab-com/blob/master/generate.rb#L64

You can verify these changes by running something like this and then inspecting test.html:

pandoc --from markdown_github-hard_line_breaks -o test.html doc/integration/twitter.md

You can also verify that GitLab continues to render the docs correctly by checking, for example, http://localhost:3000/help/integration/twitter.md