README.md: make version_specific_scratch const (#19)

I assume the reason why version_specific_scratch is a Ref{String} is so that it can
be made const, for type stability.

README.md

@@ -86,7 +86,7 @@ end
 const pkg_version = get_version()
 
 # This will be filled in by `__init__()`; it might change if we get deployed somewhere
-version_specific_scratch = Ref{String}()
+const version_specific_scratch = Ref{String}()
 
 function __init__()
     # This space will be unique between versions of my package that different major and