repair some misspelling (#1248)

* Update calloption.go

* Update random.go

repair   misspelling

transport/http/balancer/random/random.go

@@ -27,7 +27,7 @@ func (b *Balancer) Pick(ctx context.Context) (node *registry.ServiceInstance, do
 	b.lock.RUnlock()
 
 	if len(nodes) == 0 {
-		return nil, nil, fmt.Errorf("no instances avaiable")
+		return nil, nil, fmt.Errorf("no instances available")
 	}
 	if len(nodes) == 1 {
 		return nodes[0], func(context.Context, balancer.DoneInfo) {}, nil

transport/http/calloption.go

@@ -94,7 +94,7 @@ func Header(header *http.Header) CallOption {
 	return HeaderCallOption{header: header}
 }
 
-// HeaderCallOption is retrive response header for client call
+// HeaderCallOption is retrieve response header for client call
 type HeaderCallOption struct {
 	EmptyCallOption
 	header *http.Header