CONTRIBUTING.md: swap revision with version

The use of "revision" makes no sense for the first version
since it wasn't "revised" from anything. Nothing existed prior.

Change-Id: Ie82a03a3786e0ce7261a89032d2d3ebb12dcf4c0
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/221025
Reviewed-by: Damien Neil <dneil@google.com>

CONTRIBUTING.md

@@ -2,7 +2,7 @@
 
 Go protocol buffers is an open source project and accepts contributions.
 
-This project is the first major revision of Go protobufs,
+This project is the first major version of Go protobufs,
 while the next major revision of this project is located at
 [protocolbuffers/protobuf-go](https://github.com/protocolbuffers/protobuf-go).
 Most new development effort is focused on the latter project,