Update README.md

By default, options.allowMultipleHeaders if false if not specified. Fixing mistake in example code which says that it is true by default.

Checked source code and found this to confirm (lib/index.js: 13).

"options.allowMultipleHeaders = options.allowMultipleHeaders === true ? true : false;"
Merge pull request #11 from agchou/patch-1

Update README.md