docs: use lowercase for network names

[Tryphonpatta]

Summary

just a little change in readme for user's understanding and correction the wrong network ( first Upper case)

Changes

Change network from Amoy to Amoy

Issues

Readme.md

[HJunyuan]

Thanks for raising this PR!

[HJunyuan]

This PR is being blocked by 2 issues:

1. Failing npm run test:
   • Previously fixed in this PR (fix: accept application/ld+json when fetching context open-attestation#307) when open-attestation couldn't fetch JSON-LD contexts properly
   • Pending the merge of fix: bump @govtechsg/open-attestation from 6.4.1 to 6.9.5 #317
2. Commit lint error:
   • Would require renaming this commit to docs: use lowercase for network names (missing docs prefix)