Use 'a 03' instead of 'an 03'

Author: tcharding

09_3_Testing_a_Bitcoin_Script.md

@@ -96,7 +96,7 @@ script  |  stack
 --------+--------
         | 03
 ```
-And that's where our script ends, with nothing more to execute and an `03` sitting on top of our stack as the result of the Script.
+And that's where our script ends, with nothing more to execute and a `03` sitting on top of our stack as the result of the Script.
 
 > :note: **NOTE:** `btcdeb` allows you to repeat the previous command by hitting enter. We will be doing this in subsequent examples, so don't be surprised about `btcdeb>` prompts with nothing as input. It is simply repeating the previous (often `step`) command.