CApiFFI is available since 7.6, not 7.10

Summary

GHC user's guide says that CApiFFI language extension was added in 7.10: https://gitlab.haskell.org/ghc/ghc/-/blob/master/docs/users_guide/exts/ffi.rst#L390-393 This does not seem correct, because according to https://downloads.haskell.org/~ghc/7.6.3/docs/html/users_guide/ffi.html#ffi-capi CApiFFI is available since 7.6.

Proposed improvements or changes

Change 7.10 to 7.6 in ffi.rst.

(BTW the typo leaked into https://www.haskell.org/ghc/blog/20210709-capi-usage.html as well)

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