fix obsolete startup script name

Author: akkartik

index.md

@@ -45,7 +45,7 @@ title: Arc Programming Language
     <pre style='margin:0 auto; text-align:left'>
 <span style='color:grey'>$</span> git clone <a href='http://github.com/arclanguage/anarki'>http://github.com/arclanguage/anarki</a>
 <span style='color:grey'>$</span> cd anarki
-<span style='color:grey'>$</span> ./arc.sh
+<span style='color:grey'>$</span> ./arc
 <span style='color:grey'>arc&gt;</span> (help do)  <span style='color:blue'># online help</span>
 [mac] (do . args)
 Evaluates each expression in sequence and returns the result