effective_go: s/usr/home/ in example

When I was a lad (and well past), users were in /usr;
now they're somewhere else, I'm told.
Kids today.

Fixes #4122.

R=golang-dev, dsymonds, rsc
CC=golang-dev
https://golang.org/cl/6552060

doc/effective_go.html

@@ -1774,7 +1774,7 @@ func init() {
         log.Fatal("$USER not set")
     }
     if HOME == "" {
-        HOME = "/usr/" + USER
+        HOME = "/home/" + USER
     }
     if GOROOT == "" {
         GOROOT = HOME + "/go"