chore: rename internal variable to main in @stdlib/datasets/month-names-en

Renames the internal `lib/index.js` binding from `months` to `main`,
matching the canonical stdlib `lib/index.js` shape (~92% conformance
across 5028 lib/index.js files; 53/57 conformance within
`@stdlib/datasets`). The variable is local to the file and has no
observable effect on the public API.

Author: claude

lib/node_modules/@stdlib/datasets/month-names-en/lib/index.js

@@ -32,9 +32,9 @@
 
 // MODULES //
 
-var months = require( './main.js' );
+var main = require( './main.js' );
 
 
 // EXPORTS //
 
-module.exports = months;
+module.exports = main;