fix doc comment typo (#4954)

src/core/instance/state.js

@@ -58,7 +58,7 @@ function initProps (vm: Component, propsOptions: Object) {
   const propsData = vm.$options.propsData || {}
   const props = vm._props = {}
   // cache prop keys so that future props updates can iterate using Array
-  // instead of dyanmic object key enumeration.
+  // instead of dynamic object key enumeration.
   const keys = vm.$options._propKeys = []
   const isRoot = !vm.$parent
   // root instance props should be converted