[docs][llvm-strings] Clarify "printable character" wording

The --bytes option uses the phrase "printable ASCII characters", but the
description section used simply "printable characters". To avoid any
confusion about locale impacts etc, this change adopts the former's
phrasing in both places. It also fixes a minor grammar issue in the
description.

Reviewed by: MaskRay

Differential Revision: https://reviews.llvm.org/D68016

llvm-svn: 372865