benchmark: update obsolete information pointer

A doc suggested in an error message is no longer the place to get the
information about required http benchmarkers. Update the error message
to point to the current location for the information.

PR-URL: https://github.com/nodejs/node/pull/12026
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Franziska Hinkelmann <franziska.hinkelmann@gmail.com>

benchmark/_http-benchmarkers.js

@@ -88,9 +88,9 @@ exports.run = function(options, callback) {
     benchmarker: exports.default_http_benchmarker
   }, options);
   if (!options.benchmarker) {
-    callback(new Error('Could not locate any of the required http ' +
-                       'benchmarkers. Check benchmark/README.md for further ' +
-                       'instructions.'));
+    callback(new Error('Could not locate required http benchmarker. See ' +
+                       'https://github.com/nodejs/node/blob/master/doc/guides/writing-and-running-benchmarks.md##http-benchmark-requirements ' +
+                       'for further instructions.'));
     return;
   }
   const benchmarker = benchmarkers[options.benchmarker];