[llvm-gsymutil] Fix command descriptions introduced in upstream D148775 when switching to OptTable

[D148775](https://reviews.llvm.org/D148775) changed the help msg for `--addresses-from-stdin` option when switching to OptTable

This patch also fixed a small problem in the argument type error msg

Test Plan: NFC

Differential Revision: https://reviews.llvm.org/D153086