cmd/go: fix minor typo

LGTM=bradfitz
R=golang-codereviews, bradfitz
CC=golang-codereviews
https://golang.org/cl/170770043

src/cmd/go/doc.go

@@ -247,7 +247,7 @@ The arguments are space-separated tokens or double-quoted strings
 passed to the generator as individual arguments when it is run.
 
 Quoted strings use Go syntax and are evaluated before execution; a
-quoted string appears a single argument to the generator.
+quoted string appears as a single argument to the generator.
 
 Go generate sets several variables when it runs the generator: