Fix filename of md file for en-US version (#3600)

Initially it was `en-us`, but the folder is `en-US` .

When creating a new component, the build doesn't work because it hopes a file in the `en-US` folder.

build/bin/new.js

@@ -74,7 +74,7 @@ export default {
     content: `## ${chineseName}`
   },
   {
-    filename: path.join('../../examples/docs/en-us', `${componentname}.md`),
+    filename: path.join('../../examples/docs/en-US', `${componentname}.md`),
     content: `## ${componentname}`
   },
   {