Fix "go get" command in README file.
Fix "go get" command in README file.

Use the same command line as used by the continue_install script, which
is:

go get -u skia.googlesource.com/buildbot.git/perf/go/...

BUG=None
TEST=None
R=stephana@google.com, jcgregorio@google.com

Author: tfarina@chromium.org

Review URL: https://codereview.chromium.org/597303003

Tiger66639/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,Tiger66639/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,Tiger66639/skia-buildbot,Tiger66639/skia-buildbot,Tiger66639/skia-buildbot,Tiger66639/skia-buildbot,Tiger66639/skia-buildbot,google/skia-buildbot