minor clean-ups (#5701)

lib/propmangle.js

@@ -188,7 +188,7 @@ function mangle_properties(ast, options) {
     var regex = options.regex;
 
     // note debug is either false (disabled), or a string of the debug suffix to use (enabled).
-    // note debug may be enabled as an empty string, which is falsey. Also treat passing 'true'
+    // note debug may be enabled as an empty string, which is falsy. Also treat passing 'true'
     // the same as passing an empty string.
     var debug = options.debug !== false;
     var debug_suffix;