Broken link to unused linter in README

[cbandy]

EnabledByDefaultLinters section of README links to https://github.com/dominikh/go-tools/tree/master/cmd/unused which was removed in dominikh/go-tools@a1e4860.

(The commit message there makes it sound like the unused functionality has changed.)

[vovapi]

It seems that the fork of go-tools (https://github.com/golangci/go-tools) is actually used:

golangci-lint/go.mod

Line 18 in 2e8d832

github.com/golangci/go-tools v0.0.0-20190318055746-e32c54105b7c

I think README should point there.