chore: update scripts example to new GetElem syntax (leanprover/lake#171)

README.md

@@ -319,7 +319,7 @@ Greet the entity with the given name. Otherwise, greet the whole world.
 -/
 script greet (args) do
   if h : 0 < args.length then
-    IO.println s!"Hello, {args.get 0 h}!"
+    IO.println s!"Hello, {args[0]'h}!"
   else
     IO.println "Hello, world!"
   return 0

examples/scripts/lakefile.lean

@@ -14,7 +14,7 @@ Greet the entity with the given name. Otherwise, greet the whole world.
 @[default_script]
 script greet (args) do
   if h : 0 < args.length then
-    IO.println s!"Hello, {args.get ⟨0, h⟩}!"
+    IO.println s!"Hello, {args[0]'h}!"
   else
     IO.println "Hello, world!"
   return 0