docs: fix guide on enabling source map generation for iOS

[floydkim]

This PR fixes the guide for generating source maps during iOS builds.

Following the current guide as-is does not generate a source map file.
This diff adds export to the environment variable declaration so that child processes can read the SOURCEMAP_FILE variable.

If child processes can't read the SOURCEMAP_FILE, react-native-xcode.sh script will not recognize it, and -output-source-map option will not be added when executing hermesc.
As a result, the source map file is not generated.

export-sourcemap-env.mov

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[Simek]

Hey @floydkim, thanks for spotting that issue and providing a correction! 🙏

Can you update also newly cut 0.82 docs file? Can be in a separate PR if you want, just LMK. This one is fine to ship otherwise!

[Simek]

Thanks for the quick update, let's ship it! 🚀

[floydkim]

@Simek Wow. I was going to let you know after the GitHub checks were finished, but you saw it already!
Thank you as always for your hard work.