Add readable, writable and duplex

snippets/isStream.md

@@ -2,7 +2,7 @@
 
 Checks if the given argument is a stream.
 
-Check if the value is different from `null`, use `typeof` to check if a value is of type `object` and the `pipe` property is of type `function`.
+Check if the value is different from `null`, use `typeof` to check if the value is of type `object` and the `pipe` property is of type `function`.
 
 ```js
 const isStream = val => val !== null && typeof val === 'object' && typeof val.pipe === 'function';