Adjust applescript workaround

It turns out that there are two path types in applescript, and I had mixed two of them in my previous patch.  Annoyingly, things seemed to work when editing locally, unsure why.

(cherry picked from commit c759aa9)

Author: staticfloat

contrib/mac/app/startup.applescript

@@ -1,4 +1,3 @@
 set RootPath to (path to me)
 set JuliaPath to POSIX path of ((RootPath as text) & "Contents:Resources:julia:bin:julia")
-set JuliaFile to POSIX file JuliaPath
-do shell script "open -a Terminal '" & JuliaFile & "'"
+do shell script "open -a Terminal '" & JuliaPath & "'"