Fix argument type of fs.writeFileSync (#1075)

It causes error in Node version 14+.
A number type is not valid for the second argument
of fs.writeFileSync from the documentation.
https://nodejs.org/docs/latest-v14.x/api/fs.html#fs_fs_writefilesync_file_data_options

bin/monitor

@@ -10,7 +10,7 @@ var fs = require('fs'),
 // Write the pidFile to disk for later use
 //
 function writePid(file, pid) {
-  fs.writeFileSync(file, pid, 'utf8');
+  fs.writeFileSync(file, String(pid), 'utf8');
 }
 
 //
@@ -84,4 +84,4 @@ process.on('message', function (data) {
     started = true;
     start(options);
   }
-});
+});