Fixes to the Get Started Workbox guide (#7820)
Fixes to the Get Started Workbox guide (#7820)

In the *Get Started* Workbox guide,

- Changed `new RegExp('.*\.js')` to `/\.js$/` when matching JavaScript files so that the regular expression is consistent with the one written in a previous code snippet. Also, `new RegExp('.*\.js')` would be problematic because it does not escape the backslash (effectively saying `/.*.js/`) and even if it did escape the backslash, it would match not only `.js` files, but also`.json` files.
- Added missing punctuation
- Added missing determiner
- Fixed some typos
lucab85/WebFundamentals,myakura/WebFundamentals,google/WebFundamentals,myakura/WebFundamentals,google/WebFundamentals,lucab85/WebFundamentals,lucab85/WebFundamentals,google/WebFundamentals,myakura/WebFundamentals,google/WebFundamentals,myakura/WebFundamentals,lucab85/WebFundamentals