Fix grammar typo in Design Principles docs (#9324)

features → feature
> There is nothing "bad" about using state or lifecycle hooks in components. Like any powerful feature**s**, they should be used in moderation, but we have no intention to remove them.

I can’t explain the exact grammatical principle this violates, but it sounds wrong to my native English ears that “feature” is plural here. Another way to check if the grammar sounds right is to change the order of the clauses: ”They should be used in moderation, like any powerful feature“ sounds right, whereas “They should be used in moderation, like any powerful features” does not.

contributing/design-principles.md

@@ -20,7 +20,7 @@ The key feature of React is composition of components. Components written by dif
 
 For example, it should be possible to introduce some local state into a component without changing any of the components using it. Similarly, it should be possible to add some initialization and teardown code to any component when necessary.
 
-There is nothing "bad" about using state or lifecycle hooks in components. Like any powerful features, they should be used in moderation, but we have no intention to remove them. On the contrary, we think they are integral parts of what makes React useful. We might enable [more functional patterns](https://github.com/reactjs/react-future/tree/master/07%20-%20Returning%20State) in the future, but both local state and lifecycle hooks will be a part of that model.
+There is nothing "bad" about using state or lifecycle hooks in components. Like any powerful feature, they should be used in moderation, but we have no intention to remove them. On the contrary, we think they are integral parts of what makes React useful. We might enable [more functional patterns](https://github.com/reactjs/react-future/tree/master/07%20-%20Returning%20State) in the future, but both local state and lifecycle hooks will be a part of that model.
 
 Components are often described as "just functions" but in our view they need to be more than that to be useful. In React, components describe any composable behavior, and this includes rendering, lifecycle, and state. Some external libraries like [Relay](http://facebook.github.io/relay/) augment components with other responsibilities such as describing data dependencies. It is possible that those ideas might make it back into React too in some form.