reconciler docs: fix small typo - "mode" (instead of "node") (#25863)

Author: satelllte

packages/react-reconciler/README.md

@@ -81,7 +81,7 @@ const HostConfig = {
 }
 ```
 
-If your target platform has immutable trees, you'll want the **persistent mode** instead. In that mode, existing nodes are never mutated, and instead every change clones the parent tree and then replaces the whole parent tree at the root. This is the node used by the new React Native renderer, codenamed "Fabric".
+If your target platform has immutable trees, you'll want the **persistent mode** instead. In that mode, existing nodes are never mutated, and instead every change clones the parent tree and then replaces the whole parent tree at the root. This is the mode used by the new React Native renderer, codenamed "Fabric".
 
 ```js
 const HostConfig = {