Fix credentials_file flag in README

README.md

diff --git a/README.md b/README.md
index 619efd8..f8e4995 100644
--- a/README.md
+++ b/README.md
@@ -114,8 +114,9 @@
     -server_addr string
         Your http server's address in the format of host:port
         (default "example.com:80")
-    -username_password string
-        (eg 'username:password') credentials for your server. Leave blank to bypass authentication.
+    -credentials_file string
+        File containing credentials for your server. Leave blank to bypass authentication.
+        File should have exactly one line of the form 'username:password'.
     -ca_file string
         Absolute path to your server's Certificate Authority root cert. Downloading all roots currently recommended by the Google Internet Authority is a suitable alternative https://pki.google.com/roots.pem. Leave blank to connect using http rather than https.
     -full_server_name string