net/rpc: add missing import to rpc server documentation

Change-Id: Idca6115181960eed7a955027ee77a02decb4e7f2
Reviewed-on: https://go-review.googlesource.com/31179Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/net/rpc/server.go

@@ -55,6 +55,8 @@
 
 		package server
 
+		import "errors"
+
 		type Args struct {
 			A, B int
 		}