doc: add question mark to LEAN_IDENT_RE

[lovettchris]

See https://leanprover.github.io/lean4/doc/functions.html

Without this fix:

The question mark is highlighted in a way that sends a confusing message to the user that the question mark is NOT part of the name - but it is just part of the name.

With this fix:

[digama0]

You added ? as an initial identifier character though, which is not correct: e.g. ?a is not an identifier (it is a named metavariable). ? is legal as a trailing identifier character.

[lovettchris]

Woops good catch - I pushed a fix.