docs: add missing context package in sample code (#566)

Signed-off-by: WANG Xu <feici02@outlook.com>

Author: feici02

go/README.md

@@ -25,6 +25,7 @@ go run chat.go
 package main
 
 import (
+	"context"
     "fmt"
     "log"
 
@@ -274,6 +275,7 @@ Enable streaming to receive assistant response chunks as they're generated:
 package main
 
 import (
+	"context"
     "fmt"
     "log"