fix multi-line string in showvalues

[islent]

Fix https://github.com/timholy/ProgressMeter.jl/issues/224?notification_referrer_id=NT_kwDOAdnnorMyNjY0Nzk3OTgzOjMxMDU3ODI2#issuecomment-1370681492

However, I cannot figure out why there is one additional blank line while displaying Dict:

p = ProgressMeter.Progress(10)
for iter in 1:10
    sleep(0.1)
    next!(p; showvalues = Dict(:1 => 1, :halfdone => (1 >= 50/2)))
end