Update help example to use vars

[alex-broad]

Fixes #426

[alecthomas]

I think this would be better in the README, there's no need to add it to an arbitrary example.

[alex-broad]

As discussed in #426, others have come to the help example code. The README already has discussion of variable interpolation. Why not both? I'd argue the reason people are looking at specific examples is that the README is already pretty long and covers a lot; examples are usually more concise and directed.

At the very least it's worth removing the incorrect func (f *flagWithHelp) Help() string as the HelpProvider interface explicitly only supports args/commands.

[alecthomas]

Seems reasonable.