text/template: add an if func example

text/template: add an if func example

Updates #13880

Change-Id: I0fd3b1a32e485bc8f15238c86a8bbdc161f20fa5
GitHub-Last-Rev: d1e92b31243c86435add422ee46aaff768453cad
GitHub-Pull-Request: golang/go#71922
Reviewed-on: https://go-review.googlesource.com/c/go/+/651916
Auto-Submit: Ian Lance Taylor <iant@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: Junyang Shao <shaojunyang@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>