internal/coverage: fix comment dupword, error typo

internal/coverage: fix comment dupword, error typo

- Correct duplicated word in comments.
- Fix typo in error message.

Change-Id: I688d723ea3ac4d0b1981afd747e4b2df00c81448
Reviewed-on: https://go-review.googlesource.com/c/go/+/485016
Reviewed-by: David Chase <drchase@google.com>
Run-TryBot: Than McIntosh <thanm@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>