cmd/go: trivial help message fix for go help get

Direct reference to go help build where the flags are described.

R=golang-dev, adg, rsc
CC=golang-dev
https://golang.org/cl/5825046

Author: vdobler

src/cmd/go/doc.go

@@ -213,7 +213,7 @@ Get downloads and installs the packages named by the import paths,
 along with their dependencies.
 
 The -a, -n, -v, -x, and -p flags have the same meaning as in 'go build'
-and 'go install'.  See 'go help install'.
+and 'go install'.  See 'go help build'.
 
 The -d flag instructs get to stop after downloading the packages; that is,
 it instructs get not to install the packages.

src/cmd/go/get.go

@@ -24,7 +24,7 @@ Get downloads and installs the packages named by the import paths,
 along with their dependencies.
 
 The -a, -n, -v, -x, and -p flags have the same meaning as in 'go build'
-and 'go install'.  See 'go help install'.
+and 'go install'.  See 'go help build'.
 
 The -d flag instructs get to stop after downloading the packages; that is,
 it instructs get not to install the packages.