maint: convert `...' to '...' in --help output

maint: convert `...' to '...' in --help output

All affected lines end with \ or \n\, so run this command
until it produces no new changes (4 times):
  git grep -E -l '`[^ ]+'\''.*\\' src \
    |xargs perl -pi -e 's/`([^ ]+'\''.*\\)/'\''$1/'