Add --shell to fnm env commands in the README

[mo8it]

When specifying --shell, the fnm env command takes about 1.3ms instead of 54ms on my machine. This is about 40x speedup.

The README has a section for every shell anyway, so adding the option saves some time on shell initialization.

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Latest commit: c8796b5

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Name	Status	Preview	Comments	Updated (UTC)
fnm	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	Aug 11, 2024 11:50am

[Schniz]

That's the spirit! Thank you.