[libpng16] Fixed typo in example.c (png_free_image should be png_image_free)

(Bug report by John Smith)
diff --git a/example.c b/example.c
index a71000f..99d2f05 100644
--- a/example.c
+++ b/example.c
@@ -114,13 +114,13 @@
 
          else
          {
-            /* Calling png_free_image is optional unless the simplified API was
+            /* Calling png_image_free is optional unless the simplified API was
              * not run to completion.  In this case if there wasn't enough
              * memory for 'buffer' we didn't complete the read, so we must free
              * the image:
              */
             if (buffer == NULL)
-               png_free_image(&image);
+               png_image_free(&image);
 
             else
                free(buffer);