docs: remove main field from helia-101 example

[RenatoPerotti]

For me the helia-101 example did not work, because of the package.json referring to the index.js file, I suppose the one in the test directory (./test/index.spec.js) is the one we should be referring to!?

[achingbrain]

Thanks for opening this and sorry it's taken so long to get to it.

The fix here would be to remove the "main" field, it's not necessary for this example.