use lower case

[rikkix]

If not, it cannot be imported in another package which uses go mod, or it would be

go: finding module for package github.com/Sirupsen/logrus
go: found github.com/Sirupsen/logrus in github.com/Sirupsen/logrus v1.6.0
go: found github.com/crosbymichael/tracker/registry/inmem in github.com/crosbymichael/tracker v0.0.0-20160805174527-d54763ad6ba1
go: github.com/xxxx/xxxx imports
        github.com/Sirupsen/logrus: github.com/Sirupsen/[email protected]: parsing go.mod:
        module declares its path as: github.com/sirupsen/logrus
                but was required as: github.com/Sirupsen/logrus

Cause logrus declares it as github.com/sirupsen/logrus, so importing github.com/Sirupsen/logrus is invalid
https://github.com/sirupsen/logrus/blob/6699a89a232f3db797f2e280639854bbc4b89725/go.mod#L1

Related PR: sirupsen/logrus#384