doc(i18n): typo (#491)

i18next--backend -> i18next-xhr-backend

Author: HIRANO-Satoshi

current/en-us/7. plugins/4. i18n.md

@@ -138,7 +138,7 @@ Fourth, in those subfolders create a file named `translation.json` which contain
 ```
 
 Fifth, create (if you haven't already) a file `main.js` in your `src` folder to configure the plugin. 
-We use i18next-http-backend here. Note that, since i18next-xhr-backend is deprecated, Backend in aurelia-i18n and Backend in i18next--backend are not valid options.
+We use i18next-http-backend here. Note that, since i18next-xhr-backend is deprecated, Backend in aurelia-i18n and Backend in i18next-xhr-backend are no longer valid options.
 
 
 > Info