@uppy/xhr-upload: fix type in README.md (#4416)

Fix README.md

packages/@uppy/xhr-upload/README.md

@@ -18,7 +18,7 @@ import Uppy from '@uppy/core'
 import XHRUpload from '@uppy/xhr-upload'
 
 const uppy = new Uppy()
-uppy.use(Uppy, {
+uppy.use(XHRUpload, {
   // Options
 })
 ```