Fix propagator package in release.go

[damemi]

Got a little hot on the merge button in #539 and didn't notice the weird changes in https://github.com/GoogleCloudPlatform/opentelemetry-operations-go/pull/539/files#diff-5e69ba298fd881f2cf26d39a12cf8ac7f4df5900cb66971179941ad33582f93e

Luckily, make release failed with the following:

go run tools/release.go prepare
go: errors parsing example/trace/http/go.mod:
example/trace/http/go.mod:17:2: require github.com/GoogleCloudPlatform/opentelemetry-operations-go/propagator: version "v" invalid: must be of the form v1.2.3
2022/11/29 16:25:36 running "/usr/lib/google-golang/bin/go mod edit -require=github.com/GoogleCloudPlatform/opentelemetry-operations-go/internal/resourcemapping@v0.34.2 example/trace/http/go.mod": exit status 1
exit status 1
make: *** [Makefile:207: prepare-release] Error 1

I don't know exactly how adding the / fixes this, but I noticed the other modules all had a trailing slash so I assume there is some text manipulation in the script that depends on it.