design/34481-opencoded-defers.md: fix some typos and make some clarifications
design/34481-opencoded-defers.md: fix some typos and make some clarifications

Fixes based on comments from Brad and Bryan and others.

Change-Id: I64899046023c01fc549c05ca6ab0900269758425
Reviewed-on: https://go-review.googlesource.com/c/proposal/+/197057
Reviewed-by: Bryan C. Mills <1c8aad60184261ccede67f5c63a0d2a3bf3c9ff4@google.com>
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

golang/proposal,golang/proposal