doc/go_spec.html: update type identity example
doc/go_spec.html: update type identity example

In the Type identity section, the example provides various types as givens.

The example refers to the type *T5, but it is not provided in the givens.

I am assuming this was a typo, and was meant to refer to *A1 or *B1.
*B1 seems to be in alignment with the rest of the provided examples.

Change-Id: I554319ee8bca185c3643559321417e8b2a544ba0
GitHub-Last-Rev: e80560d32abcb36c51554606beb09a53d45f0062
GitHub-Pull-Request: golang/go#52143
Reviewed-on: https://go-review.googlesource.com/c/go/+/398075
Reviewed-by: Robert Griesemer <a6752097faebf1f02d80107d77a9be009089afe3@golang.org>
Trust: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>

golang/go,golang/go,CAFxX/go,nawawi/go,golang/go,golang/go,nawawi/go,golang/go,CAFxX/go,golang/go,CAFxX/go,CAFxX/go,CAFxX/go,nawawi/go,CAFxX/go,nawawi/go,nawawi/go,nawawi/go,nawawi/go,nawawi/go,golang/go,CAFxX/go,CAFxX/go,golang/go