tools: fix error in custom ESLint rule

Fix previously-unnoticed typo in `required-modules.js`.

Refs: nodejs#13758 (comment)
PR-URL: nodejs#13758
Reviewed-By: Teddy Katz <[email protected]>
Reviewed-By: Vse Mozhet Byt <[email protected]>
Reviewed-By: Colin Ihrig <[email protected]>
Reviewed-By: James M Snell <[email protected]>
Reviewed-By: Michael Dawson <[email protected]>

tools/eslint-rules/required-modules.js

@@ -75,7 +75,7 @@ module.exports = function(context) {
       if (foundModules.length < requiredModules.length) {
         var missingModules = requiredModules.filter(
           function(module) {
-            return foundModules.indexOf(module === -1);
+            return foundModules.indexOf(module) === -1;
           }
         );
         missingModules.forEach(function(moduleName) {