README: correct flag value in example

Before this change, localperf would log:

localperf: 2017/05/09 14:10:47 failed to fetch recent uploads: Get localhost:8081/uploads?extra_label=by&extra_label=upload-time&limit=16: unsupported protocol scheme "localhost"

Change-Id: Ifeba048c9dd54cf2c02ed539c5bfa0e6074fd60a
Reviewed-on: https://go-review.googlesource.com/43032Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

README.md

@@ -22,7 +22,7 @@ the complete stack on your machine with an in-memory datastore.
 go get -u golang.org/x/perf/storage/localperfdata
 go get -u golang.org/x/perf/analysis/localperf
 localperfdata -addr=:8081 -view_url_base=http://localhost:8080/search?q=upload: &
-localperf -addr=:8080 -storage=localhost:8081
+localperf -addr=:8080 -storage=http://localhost:8081
 ```
 
 The storage system is designed to have a