Indent headers in usage example

README.md

@@ -18,12 +18,12 @@ npm install --save expired
 const expired = require('expired');
 
 const headers = `
-Age: 0
-Cache-Control: public, max-age=300
-Content-Encoding: gzip
-Content-Type: application/json;charset=utf-8
-Date: Fri, 23 Dec 2016 05:50:31 GMT
-Last-Modified: Fri, 23 Dec 2016 05:23:23 GMT`;
+  Age: 0
+  Cache-Control: public, max-age=300
+  Content-Encoding: gzip
+  Content-Type: application/json;charset=utf-8
+  Date: Fri, 23 Dec 2016 05:50:31 GMT
+  Last-Modified: Fri, 23 Dec 2016 05:23:23 GMT`;
 
 expired(headers);
 // false