Lowercase filenames to match default slugs

Currently, all slugs are forced to lowercase on Readme.com.
However when syncing, it left the name as uppercase, which **always**
resulted in a 404, and thus a new page being created.

src/cmds/docs/index.js

@@ -93,8 +93,8 @@ exports.run = function (opts) {
     files.map(async filename => {
       const file = await readFile(path.join(folder, filename), 'utf8');
       const matter = frontMatter(file);
-      // Stripping the markdown extension from the filename
-      const slug = filename.replace(path.extname(filename), '');
+      // Stripping the markdown extension from the filename and lowecasing to get the default slug
+      const slug = filename.replace(path.extname(filename), '').toLowerCase();
       const hash = crypto.createHash('sha1').update(file).digest('hex');
 
       return request