Fix markdown formatting

[nurupo]

GitHub changed how their Markdown parser works in non-backwards-compatible way, breaking custom anchors and #Headings (it's now required to have a space after #).

There are lines with ####### in https://github.com/TokTok/c-toxcore/blob/master/docs/updates/Symmetric-NAT-Transversal.md. Not sure if they were rendered to anything, but they are not being rendered right now, which seems fine to me, so I haven't done anything about them.

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This change is 

[robinlinden]

Reviewed 3 of 3 files at r1.
Review status: all files reviewed at latest revision, 1 unresolved discussion.

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INSTALL.md, line 22 at r1 (raw file):

    - [libtoxav](#libtoxav)
  - [Bootstrap daemon](#bootstrap-daemon)
  - [nTox](#ntox)

s/ntox/ntox-test-cli

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Comments from Reviewable

[iphydf]

I put some comments, but they are just about fixing more markdown formatting, nothing about the current state. This is an improvement, so I'm happy after you fix @robinlinden's comment.

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Review status: all files reviewed at latest revision, 3 unresolved discussions.

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docs/av_api.md, line 11 at r1 (raw file):

function initializes sample phone. _listen_port and _send_port are variables only meant

`_listen_port` and the same for all other code fragments in the text.

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docs/av_api.md, line 16 at r1 (raw file):

Phone requires one msi session and two rtp sessions ( one for audio and one for

Remove space after (.

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Comments from Reviewable

[nurupo]

Review status: all files reviewed at latest revision, 3 unresolved discussions, some commit checks failed.

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INSTALL.md, line 22 at r1 (raw file):

Previously, robinlinden (Robin Lindén) wrote…

s/ntox/ntox-test-cli

Nice catch.

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docs/av_api.md, line 11 at r1 (raw file):

Previously, iphydf wrote…

`_listen_port` and the same for all other code fragments in the text.

In this PR I aim to fix markdown files that have rendered correctly before but because of GitHub changing its markdown parser no longer render correctly. Changes like the one you ask me here are out of scope of what I want to do in this PR.

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Comments from Reviewable

[nurupo]

Review status: 2 of 3 files reviewed at latest revision, 3 unresolved discussions, some commit checks failed.

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INSTALL.md, line 22 at r1 (raw file):

Previously, nurupo wrote…

Nice catch.

Should be fixed now.

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Comments from Reviewable

[robinlinden]

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Reviewed 1 of 1 files at r2.
Review status: all files reviewed at latest revision, 2 unresolved discussions, some commit checks failed.

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Comments from Reviewable

[iphydf]

@nurupo, can you rebase and merge? You have merge rights, so just go ahead (and change the milestone to 0.1.8 if you do it before that's released).

[nurupo]

Rebased. @robinlinden you can merge now!