doc/articles/wiki: use correct variable name in closures guide

Fixes non-existent variable TitleValidator to be validPath in
the closures, functions literal section.

Fixes #36779

Change-Id: I59762c358c3e00d1cc03d9d1e2aace03f145321d
GitHub-Last-Rev: a5e9b17a3707fc48c6d87bab3a4968f2ef63ab65
GitHub-Pull-Request: golang/go#36783
Reviewed-on: https://go-review.googlesource.com/c/go/+/216479


Reviewed-by: Emmanuel Odeke <emm.odeke@gmail.com>