[docs] Make --version text more correct

Follow-up to r371983. Referring to "this program" in the description of
the --version option in the documentation isn't exactly correct, because
the docs are not part of the program, and so "this program" doesn't
really refer to anything. This patch brings the other users of this
terminology into line with the new updates to llvm-size and
llvm-strings.

Reviewed by: alexshap, MaskRay

Differential Revision: https://reviews.llvm.org/D67618

llvm-svn: 372107