Fix "ng help" typo in README.md

[OlivierCroisier]

Documentation says "ng --help", but the actual command is "ng help".

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[OlivierCroisier]

I signed it!

[googlebot]

CLAs look good, thanks!

[grizzm0]

Dupe of #3159. Please search through PRs before submitting a new one.

[filipesilva]

Closing as a dupe, but thank you for taking the time to make the PR.

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