fix setInterval usage in mobx examples to work in firefox (#3769)

examples/with-mobx-state-tree/store.js

@@ -14,7 +14,7 @@ const Store = types
         // mobx-state-tree doesn't allow anonymous callbacks changing data
         // pass off to another action instead
         self.update()
-      })
+      }, 1000)
     }
 
     function update () {

examples/with-mobx/store.js

@@ -14,7 +14,7 @@ class Store {
     this.timer = setInterval(() => {
       this.lastUpdate = Date.now()
       this.light = true
-    })
+    }, 1000)
   }
 
   stop = () => clearInterval(this.timer)