Update golint install instructions

Summary:
These are the instructions from https://github.com/golang/lint.

The fetch location changed to golang.org/x/lint/golint from github.com/golang/lint/golint.
-u tells go to update the package and its deps if they exist.
-u Shouldn't strictly be necessary, but figured we might as well follow the instructions from golint.

Test Plan:
Enable golint without having it installed.
Ensure that the install instructions now show the new location.

Reviewers: epriestley, Blessed Reviewers

Reviewed By: epriestley, Blessed Reviewers

Subscribers: Korvin

Differential Revision: https://secure.phabricator.com/D21552