truncate.1: Minor polish

[concussious]

I was studying this manual and noticed some possible improvements.

• Rewrite the document description to fit in one std. cons. line, adding "resize" to the keywords, removing only "length" as a keyword, which I find implausible.

• Switch to human readable ls output for examples. This makes them fit on standard console, and makes it easier to understand since the example operations are in megabytes already.

• Improve English and roff grammar, eliminating a linter warning and improving rapid reader comprehension.

• Remove prompt from an example which has it, to create consistency with the rest of them.

• Tag spdx

Cc @mhorne, @sergio-carlavilla