how-to link in README broken

From breakup of how-tos daea29d

how-to link in README:

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           [how-to guide](./docs/howtos.md#installing-on-a-machine-with-no-operating-system).

should be ./docs/howtos/no-os.md

and

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your own [image](./docs/howtos.md#using-your-own-kexec-image) you can also

./docs/howtos/custom-kexec.md

and

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suit individual requirements, see the [How To Guide](./docs/howtos.md).

should be:

./docs/howtos/INDEX.md