Correct grammar (#3608)

Change 'was' to 'were'.

content/docs/strict-mode.md

@@ -55,7 +55,7 @@ Since object refs were largely added as a replacement for string refs, strict mo
 
 React used to support `findDOMNode` to search the tree for a DOM node given a class instance. Normally you don't need this because you can [attach a ref directly to a DOM node](/docs/refs-and-the-dom.html#creating-refs).
 
-`findDOMNode` can also be used on class components but this was breaking abstraction levels by allowing a parent to demand that certain children was rendered. It creates a refactoring hazard where you can't change the implementation details of a component because a parent might be reaching into its DOM node. `findDOMNode` only returns the first child, but with the use of Fragments, it is possible for a component to render multiple DOM nodes. `findDOMNode` is a one time read API. It only gave you an answer when you asked for it. If a child component renders a different node, there is no way to handle this change. Therefore `findDOMNode` only worked if components always return a single DOM node that never changes.
+`findDOMNode` can also be used on class components but this was breaking abstraction levels by allowing a parent to demand that certain children were rendered. It creates a refactoring hazard where you can't change the implementation details of a component because a parent might be reaching into its DOM node. `findDOMNode` only returns the first child, but with the use of Fragments, it is possible for a component to render multiple DOM nodes. `findDOMNode` is a one time read API. It only gave you an answer when you asked for it. If a child component renders a different node, there is no way to handle this change. Therefore `findDOMNode` only worked if components always return a single DOM node that never changes.
 
 You can instead make this explicit by passing a ref to your custom component and pass that along to the DOM using [ref forwarding](/docs/forwarding-refs.html#forwarding-refs-to-dom-components).