[docs] Fix api.md indentation

[solkaz]

Fix the list bullet points in api.md to use two spaces instead of one to render sub-points

• PR has tests / docs demo, and is linted.
• Commit and PR titles begin with [ComponentName], and are in imperative form: "[Component] Fix leaky abstraction".
• Description explains the issue / use-case resolved, and auto-closes the related issue(s) (http://tr.im/vFqem).

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[oliviertassinari]

@solkaz Thanks!