Fix usage example in README

Author: hackermd

README.md

@@ -44,6 +44,10 @@ const url = 'http://localhost:8080/dicomweb';
 const client = new DICOMwebClient.api.DICOMwebClient({url});
 const studyInstanceUID = '1.2.3.4';
 const seriesInstanceUID = '1.2.3.5';
+const searchInstanceOptions = {
+  studyInstanceUID,
+  seriesInstanceUID
+};
 client.searchForInstances(searchInstanceOptions).then((instances) => {
   const promises = []
   for (let i = 0; i < instances.length; i++) {
@@ -55,10 +59,9 @@ client.searchForInstances(searchInstanceOptions).then((instances) => {
     };
     const promise = client.retrieveInstanceMetadata(retrieveInstanceOptions).then(metadata => {
       const imageType = metadata[0]["00080008"]["Value"];
-      if ( imageType[2] !== "VOLUME") {
-        return(null);
+      if (imageType[2] === "VOLUME") {
+        return(metadata[0]);
       }
-      return(metadata[0]);
     });
     promises.push(promise);
   }