docs: Minor tweaks to docs/ios_build_instructions.md.
docs: Minor tweaks to docs/ios_build_instructions.md.

Add internal links (i.e. links to anchor) as the version of Markdown used
by gitiles generate anchors for each heading automatically.

Use ``` syntax to introduce the blocs of code so that we can specify the
syntax to use for color highlighting (shell, json, ...).

Convert enumerated list (1-, 2-, ...) to the proper format to get a nice
HTML enumerated list (i.e. 1., 2., ...).

Remove superfluous '\' that appeared in the output and always format file
names as code using `filename`.

BUG=None

Review URL: https://codereview.chromium.org/1757833002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#378723}

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