Fix multiple occurences of typo "an simple" to "a simple"

examples/audio/01-simple-playback/README.txt

@@ -1,5 +1,5 @@
 If you're running this in a web browser, you need to click the window before you'll hear anything!
 
-This example code creates an simple audio stream for playing sound, and
+This example code creates a simple audio stream for playing sound, and
 generates a sine wave sound effect for it to play as time goes on. This is the
 simplest way to get up and running with procedural sound.

examples/audio/01-simple-playback/simple-playback.c

@@ -1,5 +1,5 @@
 /*
- * This example code creates an simple audio stream for playing sound, and
+ * This example code creates a simple audio stream for playing sound, and
  * generates a sine wave sound effect for it to play as time goes on. This
  * is the simplest way to get up and running with procedural sound.
  *