fix godoc comment for Namespaces client method

Namespaces method godoc has same sentence as Operator method.
Presumably a cut-n-paste error. Fix that.

api/namespace.go

@@ -53,7 +53,7 @@ type Namespaces struct {
 	c *Client
 }
 
-// Operator returns a handle to the operator endpoints.
+// Namespaces returns a handle to the namespaces endpoints.
 func (c *Client) Namespaces() *Namespaces {
 	return &Namespaces{c}
 }