Fix section name in README.md

[ardrabczyk]

[CLAassistant]

All committers have signed the CLA.

[tomasaschan]

This change is inconsistent with how command-line is styled in the rest of this document, as well as with how other similarly joined words are styled (one-letter, top-level etc).

[ardrabczyk]

The sentence is:

"Command-line flag syntax" section below.

but the section name is:

Command line flag syntax

[tomasaschan]

A better fix would then be to change the heading to use a dash. For bonus points, make the reference an actual hyperlink :)

But the change proposed in this PR would make the document more inconsistent, not less.