chore: reword in tree-view (#57884)
### What?
Minor change to use the word `content` instead of `HTML` in the build output tree-view legend.
### Why?
The `○` symbol is also used for statically pre-rendered Route Handlers, which might not return HTML, so just clarifying the wording.
[Slack thread](https://vercel.slack.com/archives/C03KAR5DCKC/p1698844583794019)
