reference: add some variety to the examples of floating point literals

[tshepang]

This would now be the only example that makes use of an underscore

[rust-highfive]

r? @huonw

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[steveklabnik]

There's another with an underscore three lines down.

[tshepang]

It's the only non-exponent example without a dot (.) and an underscore (_) @steveklabnik