Fix suggestion to "npm start -- --reset-cache"

Summary:
As discussed in facebook/react-native#11983. The double dash is necessary to pass through the argument to node. Based on the comments [here](facebook/react-native#1924 (comment)), it looks like most people use the double dash; it's unclear whether it would do anything at all if the dashes were omitted. If anyone else has better insight, let me know!
Closes facebook/react-native#13003

Differential Revision: D4731566

Pulled By: hramos

fbshipit-source-id: 62562536db7589a03a511762117cbf0e36d3aafb

packages/metro-bundler/src/node-haste/DependencyGraph/ResolutionRequest.js

@@ -391,7 +391,7 @@ class ResolutionRequest {
               `To resolve try the following:\n` +
               `  1. Clear watchman watches: \`watchman watch-del-all\`.\n` +
               `  2. Delete the \`node_modules\` folder: \`rm -rf node_modules && npm install\`.\n` +
-              '  3. Reset packager cache: `rm -fr $TMPDIR/react-*` or `npm start --reset-cache`.'
+              '  3. Reset packager cache: `rm -fr $TMPDIR/react-*` or `npm start -- --reset-cache`.'
             );
           });
         });