Fix markdown issues raised by markdownlint

[kianmeng]

Ran against this command:

markdownlint *.md doc/*.md --disable MD013 MD034 MD010 MD051 MD024 MD029

See a detailed description of the rules is available at https://github.com/markdownlint/markdownlint/blob/master/docs/RULES.md

[awalterschulze]

I would keep the $s everywhere, but otherwise I would accept.

If @michalmuskala thinks removing the $s are fine, then I don't object to merging.