readme: update CMake source directory option

[husskade]

-H flag has been replaced in 3.13 with the official source directory flag of -S.

[google-cla]

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[sergiud]

Thanks. However, we cannot merge this PR if you don't sign the CLA.

[husskade]

@googlebot I signed it!

[husskade]

Thanks. However, we cannot merge this PR if you don't sign the CLA.

I've signed it, but it seems to check doesn't recognize it.

[sergiud]

You might need to answer the bot again. If that still does not work, I guess you'll need to create a new PR.

[husskade]

@googlebot I signed it!

[husskade]

You might need to answer the bot again. If that still does not work, I guess you'll need to create a new PR.

It worked, thanks.

[sergiud]

Could you please update your commit message and describe what actually changed and maybe why? Update README.rst as a commit message is not helpful.

[husskade]

Could you please update your commit message and describe what actually changed and maybe why? Update README.rst as a commit message is not helpful.

Done.

[sergiud]

Thanks!