react: Fix typo in comment.

react/react.d.ts

@@ -167,7 +167,7 @@ declare namespace __React {
         // React.Props<T> is now deprecated, which means that the `children`
         // property is not available on `P` by default, even though you can
         // always pass children as variadic arguments to `createElement`.
-        // In the future, if we can define its call signature conditionallly
+        // In the future, if we can define its call signature conditionally
         // on the existence of `children` in `P`, then we should remove this.
         props: P & { children?: ReactNode };
         state: S;