doc: use serial comma in modules docs

Refs: nodejs/node#11321
Refs: nodejs/node#17384

PR-URL: nodejs/node#43103
Reviewed-By: Rich Trott <[email protected]>
Reviewed-By: Luigi Pinca <[email protected]>

Author: tniessen

doc/api/modules.md

@@ -560,7 +560,7 @@ wrapper that looks like the following:
 
 By doing this, Node.js achieves a few things:
 
-* It keeps top-level variables (defined with `var`, `const` or `let`) scoped to
+* It keeps top-level variables (defined with `var`, `const`, or `let`) scoped to
   the module rather than the global object.
 * It helps to provide some global-looking variables that are actually specific
   to the module, such as: