The Keybindings section is the only place mentioning differnt log levels
and one might assume that `-1` or `-j` behave the same as the default
ncurses interface.

The latter however does not show all counters by default as the single
action flags do, hence `varnishstat -f '*overload*'` will not show
`MAIN.sc_overload` by default, with `-1` however it does.

Fix grammar while here.