go/callgraph/vta: fix package doc
go/callgraph/vta: fix package doc

A blank line prevented go doc from picking up the package doc.

Change-Id: I40ddb95d0e38ee0809a852205f399d18f9d68be3
Reviewed-on: https://go-review.googlesource.com/c/tools/+/391514
Trust: Hyang-Ah Hana Kim <d7e82c3c07f92613bcbf9a1e88f6b96c13caa3b0@gmail.com>
Run-TryBot: Hyang-Ah Hana Kim <d7e82c3c07f92613bcbf9a1e88f6b96c13caa3b0@gmail.com>
gopls-CI: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
TryBot-Result: Gopher Robot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Zvonimir Pavlinovic <490af24f3def1258d2f7e9b10b20e341f6c68320@google.com>

golang/tools,golang/tools,golang/tools,golang/tools,golang/tools