Fixes reference to output bundle in Getting Started guide

[gingi]

With no arguments, the bundle that npx webpack outputs in dist/ is main.js, not bundle.js. Confirmed with both webpack@4.0.1 (the version referenced in the guide) and webpack@4.4.1 (latest stable). This commit renames all references (bundle.js → main.js) to avoid requiring new users from specifying unnecessary parameters.

[gingi]

I now notice that bundle.js is assumed to be the output in subsequent guides. It might be worthwhile modifying the Getting Started guide to rename it from main.js to bundle.js when the config file is introduced. I'll be happy to make that change to maintain parity with the other guides.

[montogeek]

Thanks for your contribution, unfortunately it was already fixed in #1951, I advice check previous PRs before submitting yours.