Fix name mismatch in react-reconciler custom build. (facebook#24272)

Author: latin-1

packages/react-reconciler/src/forks/ReactFiberHostConfig.custom.js

@@ -64,7 +64,7 @@ export const getInstanceFromNode = $$$hostConfig.getInstanceFromNode;
 export const beforeActiveInstanceBlur = $$$hostConfig.beforeActiveInstanceBlur;
 export const afterActiveInstanceBlur = $$$hostConfig.afterActiveInstanceBlur;
 export const preparePortalMount = $$$hostConfig.preparePortalMount;
-export const prepareScopeUpdate = $$$hostConfig.preparePortalMount;
+export const prepareScopeUpdate = $$$hostConfig.prepareScopeUpdate;
 export const getInstanceFromScope = $$$hostConfig.getInstanceFromScope;
 export const getCurrentEventPriority = $$$hostConfig.getCurrentEventPriority;
 export const detachDeletedInstance = $$$hostConfig.detachDeletedInstance;