Edits to next_dfs() docstring example.

[data-pup]

Edited the code example in the next_dfs() docstring, following the same loop style as shown in dwarfdump example found here: https://github.com/gimli-rs/gimli/blob/master/examples/dwarfdump.rs#L799

[fitzgen]

Thanks @data-pup !

[data-pup]

No problem! :D

[fitzgen]

Failing CI is due to rust-lang/rust#51699 -- going to just go ahead and merge this.

[philipc]

This is an improvement over the previous example, which was a non-typical use. But I think this is still a non-typical example because if you're traversing starting at the root then it's good enough to stop when next_dfs returns None. Keeping track of the depth is more useful when you start at a specific entry.