Fixed broken link to docker docs

[ ] Submission follows the Code Contribution Guidelines
[ ] There are not any other open Pull Requests for the same update/change
[ ] Commit messages are formatted according to Model Git Commit Messages
[ ] All changes are compliant with the latest version of Go and the one prior to it
[ ] The code being submitted is commented according to the Code Documentation and Commenting section of the Code Contribution Guidelines
[ ] Any new logging statements use an appropriate subsystem and logging level
[ ] Code has been formatted with go fmt
[ ] Running go test does not fail any tests or report any vet issues
[ ] Running golint does not report any new issues that did not already exist

[ ] Code is accompanied by tests which exercise both the positive and negative (error paths) conditions (if applicable)

[ ] Code is accompanied by new tests which trigger the bug being fixed to prevent regressions

[ ] An explanation of what the changes do and why they should be included is provided
[ ] Code is accompanied by updates to tests and/or new tests for the core changes (if applicable)

decred / dcrlnd