Update README.md & docs/index.md

[erikmd]

Cc @yurug

This small PR:

• updates one link in README.md
• and is a follow-up of Fix & Document static deployment CLI support #368 (the "static deployment" doc was not mentioned in the index)

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BTW, you might want to also add other links (in another PR / commit…)? as these pages are not "indexed" for the moment:

• https://github.com/ocaml-sf/learn-ocaml/blob/master/docs/howto-practice-ocaml.md
• https://github.com/ocaml-sf/learn-ocaml/blob/master/docs/tutorials/step-0.md … step-9.md
• https://github.com/ocaml-sf/learn-ocaml/blob/master/docs/exercises_tests.md

(but regarding docs/exercises_format.md : it is already mentioned in step-0.md)

[yurug]

Thank you @erikmd!