fix upgrade guide header

UPGRADE.md

@@ -1,6 +1,6 @@
 This guide will help you update your code when upgrading from older versions of rodio.
 
-# rodio 0.20 to 0.21
+# rodio 0.20.1 or earlier to current GitHub version
 
 ## Source implementations
 - Source had a required method `current_frame_len`. In the latest version of rodio *frame* has been renamed to *span*. You will need to change every occurrence of `current_frame_len` to `current_span_len`.