README.md: update badge SVG
README.md: update badge SVG

The badge SVG is updated to the official badge for pkg.go.dev.

Change-Id: Ib9f09e1661ee18f854786098ff3528d4175cf0cb
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/275935
Trust: Julie Qiu <8d32267b6b4884cf35adeaccde2b6857ae11aace@golang.org>
Trust: Joe Tsai <7277d7402195d5c2006c89f45718cf82bb3e527f@gmail.com>
Run-TryBot: Julie Qiu <8d32267b6b4884cf35adeaccde2b6857ae11aace@golang.org>
Reviewed-by: Joe Tsai <7277d7402195d5c2006c89f45718cf82bb3e527f@gmail.com>

protocolbuffers/protobuf-go,protocolbuffers/protobuf-go