README.md: update "go get" link to use x/lint repo

Change-Id: I0d827dd750b50bf1a3da6e519f4764d4b9de0985
Reviewed-on: https://go-review.googlesource.com/96198
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

README.md

@@ -6,7 +6,7 @@ Golint is a linter for Go source code.
 
 Golint requires Go 1.6 or later.
 
-    go get -u github.com/golang/lint/golint
+    go get -u golang.org/x/lint/golint
 
 ## Usage