internal: fix version by using dashes instead of plusses

https://semver.org/

This commit will be tagged v0.3.1-alpha.

Change-Id: Ie7b783ca3040f88b8f737101c9a5b5595b4c80f4
Reviewed-on: https://code-review.googlesource.com/c/google-api-go-client/+/39691
Reviewed-by: Tyler Bui-Palsulich <tbp@google.com>

go.mod

diff --git a/go.mod b/go.mod
index d96e650..5230154 100644
--- a/go.mod
+++ b/go.mod
@@ -2,7 +2,7 @@
 
 require (
 	github.com/google/go-cmp v0.2.0
-	go.opencensus.io v0.20.1+alpha
+	go.opencensus.io v0.20.1-alpha
 	golang.org/x/lint v0.0.0-20190301231843-5614ed5bae6f
 	golang.org/x/oauth2 v0.0.0-20190226205417-e64efc72b421
 	golang.org/x/sync v0.0.0-20190227155943-e225da77a7e6