crypto/tls: fix typo in cacheEntry godoc

crypto/tls: fix typo in cacheEntry godoc

Change-Id: Idcea184a5b0c205efd3c91c60b5d954424f37679
Reviewed-on: https://go-review.googlesource.com/c/go/+/460540
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Filippo Valsorda <filippo@golang.org>
Reviewed-by: Heschi Kreinick <heschi@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Tobias Klauser <tobias.klauser@gmail.com>