docs: update npm-diff to exclude angle brackets

[ethomson]

The docs.npmjs.com parser is extremely strict about angle brackets; use
< and >.

[wraithgar]

The fact that we don't have any other errors in the rest of our markdown files makes me think we may have a consistency issue with how we type things up. I checked some other markdowns and we don't use angle brackets like this when referring to parameters in the explanations, and nowhere else do we define arrays like that Array<String>

[wraithgar]

Ah, found a good example. In the other places we use angle brackets in the explanations, we have backticked things to make them stand out. cf https://docs.npmjs.com/cli/v6/commands/npm-pack#description

[wraithgar]

Thanks @ethomson for keeping on top of this!