fix: Informations on readme

Import of FlagSwitcher:
We could re-export from the index,
to match the current doc,
but that would cause a Breaking Change.
I don't know if it's really better

Condition of flag('switcher'):
The condition does not return `undefined`
but rather `null`

packages/cozy-flags/README.md

@@ -45,10 +45,11 @@ The `FlagSwitcher` shows all flags in use and displays all
 flags in use.
 
 ```js
-import flag, { FlagSwitcher } from 'cozy-flags'
+import flag from 'cozy-flags'
+import FlagSwitcher from 'cozy-flags/dist/FlagSwitcher'
 
 if (process.env.NODE_ENV !== 'production'
-    && flag('switcher') === undefined) {
+    && flag('switcher') === null) {
     flag('switcher', true) // set default flag in dev mode
 }