Use %f with printf-style formatting, not %lf

For printf, %f and %lf are synonymous, since floats are promoted to
doubles when passed as varargs.  It's only for scanf that we need to
say "%lf" for doubles and "%f" for floats.

Apparenly, some older compilers think it's naughty to say %lf and like
to spew warnings about it.

Found by grarpamp.