DOC-2487: (6 Docs) Replace 'en' with 'en_US' in Spellchecker docs and demo (#3393)

Author: Farzad Hayat

modules/ROOT/examples/live-demos/spellcheckerpro/index.js

@@ -3,7 +3,7 @@ tinymce.init({
   plugins: 'code tinymcespellchecker link',
   toolbar: 'spellchecker language spellcheckdialog',
   height: 500,
-  spellchecker_language: 'en',
+  spellchecker_language: 'en_US',
   content_langs: [
     { title: 'English (US)', code: 'en_US' },
     { title: 'English (US Medical)', code: 'en_US', customCode: 'en_US-medical' },

modules/ROOT/pages/introduction-to-tiny-spellchecker.adoc

@@ -29,7 +29,7 @@ With {cloudname} the server-side spellchecking component is automatically config
 tinymce.init({
   selector: 'textarea',
   plugins: 'tinymcespellchecker',
-  spellchecker_language: 'en'
+  spellchecker_language: 'en_US'
 });
 ----
 
@@ -47,7 +47,7 @@ tinymce.init({
   selector: 'textarea',
   plugins: 'tinymcespellchecker',
   spellchecker_rpc_url: 'localhost/ephox-spelling',
-  spellchecker_language: 'en'
+  spellchecker_language: 'en_US'
 });
 ----