Update theme label in theme.json to use sentence casing

[jasonnrosa]

Update the theme label in the theme.json file to use sentence casing to match style guide/Canvas. Sentence casing would mean "Boilerplate theme" as opposed to title casing which would be "Boilerplate Theme"

[ckconant]

@jasonnrosa is the theme.json label considered a title? I've aways found obscurity in the canvas typography guidelines around casing:

• Sentence casing is most commonly used, with title casing being generally reserved for headings only

It is a small nit, and I personally love sentence casing, but curious of when to use title casing...

[jasonnrosa]

@ckconant we still need to chat about this one a bit more internally but the more detailed canvas writing details have:

• Headers: Title case
• Subheaders: Sentence case

In this specific case, the theme label is most often used as a subheader in app vs. an h1/main header which is why we might be leaning towards changing it to sentence casing.

[ckconant]

Totally makes sense. Like I said, just a nit.

Also, Headings* 😉

[jasonnrosa]

Closed per (https://github.com/HubSpot/cms-theme-boilerplate/pull/430)