tools: fix error in custom ESLint rule

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

Backport-PR-URL: https://github.com/nodejs/node/pull/14360
Refs: https://github.com/nodejs/node/pull/13758#discussion_r122582786
PR-URL: https://github.com/nodejs/node/pull/13758
Reviewed-By: Teddy Katz <teddy.katz@gmail.com>
Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>

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) {