doc: use const consistently

[Trott]

events.markdown uses var and const in different places seemingly
arbitrarily. This change makes it favor const when possible.

[cjihrig]

LGTM

[JungMinu]

LGTM

[pmq20]

Why setting callback = function(stream) as a const? That does not seem very conventional.

[Fishrock123]

Maybe just use a function... definition? here. (I forget if that's the correct term or not...)

[jasnell]

LGTM with @Fishrock123's suggestion

[Trott]

OK, changed that const callback = function(stream) to just function callback(stream). PTAL.

[silverwind]

LGTM, but aren't there more cases of unnecessary var in the docs?

[silverwind]

Did a quick search, lots of var in docs, which could possibly be made const but I don't really see a value in doing so as it'd likely be confusing to beginners. There's nothing inherently wrong with var.

In the same vein, one could say this change here is unnecessary, so I think I'll have to redact my LGTM.

[Trott]

@silverwind The reason why I did it for this doc in particular is that there were conflicting examples. Well, not really conflicting, but inconsistent. One used const and one used var for basically identical situations. That seemed to be potentially confusing to beginners so I went to weed out the needless inconsistency.

[Trott]

(I'd be just as happy to convert the existing const to a var for consistency, but it seems like the project tends to favor const where it can be used, so that's the direction I went instead.)

[Trott]

(One argument for var instead of const is that it will work outside of strict mode so we'd be making fewer assumptions about the user's environment.)

[Trott]

I actually find that last argument (strict mode) compelling and am closing this PR.