docs: enlarge the title used by the ios-build-instructions page
docs: enlarge the title used by the ios-build-instructions page

This patch makes the title of ios_build_instructions.md match with
the other two (mac_build_instructions.md and linux_build_instructions.md).

It also reorders/rewords the title so it matches with the Mac one.

This patch also changes the headings to use "##" rather than "____" as that is what is recommended by the style guide at https://github.com/google/styleguide/blob/gh-pages/docguide/style.md#atx-style-headings.

BUG=None
R=sdefresne@chromium.org

Review URL: https://codereview.chromium.org/1755133002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#378755}

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