README: Rename the variables in the .env file

[kerns]

I am not sure the above instructions align with vars set in the .env file after running npx sanity@latest init --env.

When I run this my .env ends up with...

PUBLIC_SANITY_PROJECT_ID="xxxxxxxx"
PUBLIC_SANITY_DATASET="production"

Question or issue is with the lack of _STUDIO_ as part of this variable. Might not be important, or might be something I've misunderstood, just seemed odd.

[christianhg]

Hi! I've now removed this bit from the README since it's unneeded. The CLI seems to create env vars with correct names, so no need to rename them: #47