FIX text -- about until

Now wording about `until` are more correct.

See: skilldrick#15

Author: ruv

index.markdown

@@ -684,8 +684,8 @@ Now hit `B`, then `C`, and you should see the following:
 
 Forth has another kind of loop called `begin until`. This works like a `while`
 loop in C-based languages. Every time the word `until` is hit, the interpreter
-checks to see if the top of the stack is non-zero (true). If it is, it jumps
-back to the matching `begin`. If not, execution continues.
+checks to see if the top of the stack is non-zero (true). If it is zero, it jumps
+back to the matching `begin`. If it is non-zero, execution continues after `until`.
 
 Here's an example of using `begin until` to print key codes: