fix(history): suffix float literals CI's newer rustc now requires

Another toolchain-drift break: rustc's upcoming float_literal_f32_fallback
lint (rust-lang/rust#154024) — now active on CI's floating stable,
still absent locally — errors on unsuffixed float literals passed to
egui_plot's Into<f32> builder methods, since f64 has no From<f64> for
f32 impl to fall back to. Every other Stroke::new(...) call in this
codebase already uses the _f32 suffix; these two (Line::width, and one
Stroke::new) were the only ones that didn't.

Refs #175