cmd/govulncheck: update doc.go
cmd/govulncheck: update doc.go

Change-Id: I8345d8344d3e87b8003e7ecf64ca401ac3117237
Reviewed-on: https://go-review.googlesource.com/c/vuln/+/427475
Auto-Submit: Julie Qiu <e6cb64861edcc38971590ff27c111e0e672de045@google.com>
Reviewed-by: Jonathan Amsterdam <e3d3698b2ccd5955e4adf250d0785062d0f9018b@google.com>
Reviewed-by: Julie Qiu <e6cb64861edcc38971590ff27c111e0e672de045@google.com>
TryBot-Result: Gopher Robot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Run-TryBot: Julie Qiu <e6cb64861edcc38971590ff27c111e0e672de045@google.com>

golang/vuln,golang/vuln