Use more common "subtract" instead of obsolete "substract" (#958)

docs/src/pages/Home/counterCode.js

@@ -5,7 +5,7 @@ app({
   view: state =>
     h("div", {}, [
       h("h1", {}, state),
-      h("button", { onclick: state => state - 1 }, "substract"),
+      h("button", { onclick: state => state - 1 }, "subtract"),
       h("button", { onclick: state => state + 1 }, "add")
     ]),
   node: document.getElementById("app")

docs/src/pages/Home/index.jsx

@@ -116,7 +116,7 @@ export default (state) => (
         }}
       >
         <h1>{state.count}</h1>
-        <button class="primary-button" onclick={Down}>substract</button>
+        <button class="primary-button" onclick={Down}>subtract</button>
         <button class="primary-button" onclick={Up}>add</button>
       </div>
     </div>