regexp: add example for NumSubexp
regexp: add example for NumSubexp

Updates #21450

Change-Id: Idf276e97f816933cc0f752cdcd5e713b5c975833
GitHub-Last-Rev: 198e585f92db6e7ac126b49cd751b333e9a44b93
GitHub-Pull-Request: golang/go#33490
Reviewed-on: https://go-review.googlesource.com/c/go/+/189138
Run-TryBot: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>

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