[clang-format] Tweak help text a bit

In particular, make it clear that `--style=file` is the default,
since there's some confusion about this, e.g. here:
https://stackoverflow.com/questions/61455148/

Differential Revision: https://reviews.llvm.org/D128984

NOKEYCHECK=True
GitOrigin-RevId: b1f0efc06acc27ea07632452d9cf50eb7cc0c7f1
1 file changed