spec: fix typo in a type parameter example
spec: fix typo in a type parameter example

Fixes #54973

Change-Id: Ibad9dd124617a1bbc23abd17cbd6e9e9928e3ed9
GitHub-Last-Rev: 1c6affb967ef925ec12420e31c6423a615f90acc
GitHub-Pull-Request: golang/go#55021
Reviewed-on: https://go-review.googlesource.com/c/go/+/430316
Reviewed-by: Cherry Mui <d62e63aa42ce272d7b6a5055d97e942b33a34679@google.com>
Run-TryBot: Daniel Martí <29bf28b42a938ace12a5819da4ed4ba03d82f315@mvdan.cc>
Reviewed-by: Daniel Martí <29bf28b42a938ace12a5819da4ed4ba03d82f315@mvdan.cc>
Reviewed-by: Robert Griesemer <a6752097faebf1f02d80107d77a9be009089afe3@google.com>
TryBot-Result: Gopher Robot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

golang/go,golang/go,golang/go,golang/go,golang/go,golang/go,golang/go,golang/go