Don't add @mathjax to package name twice

Author: dpvc

components/mjs/input/tex/extensions/mhchem/mhchem.js

@@ -1,4 +1,4 @@
 import './lib/mhchem.js';
 import {fontExtension} from '../../extension.js';
 
-fontExtension('[tex]/mhchem', '@mathjax/mathjax-mhchem-font-extension');
+fontExtension('[tex]/mhchem', 'mathjax-mhchem-font-extension');