design/32437-try-builtin.md: fixed typo, an example, and added reference
design/32437-try-builtin.md: fixed typo, an example, and added reference

- The definition of `try` used `t1` twice.
- The fmt.HandleErrorf example was incorrect.
- Added a reference to an existing `try` implementation.

Change-Id: I9d3fad554f406a539a6805e21a1866e445ea0e86
Reviewed-on: https://go-review.googlesource.com/c/proposal/+/180797
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>

golang/proposal,golang/proposal