Correct various typos in code comments

Found via `codespell -q 3`

Closes #263
diff --git a/jdmerge.c b/jdmerge.c
index d67faec..b3fec04 100644
--- a/jdmerge.c
+++ b/jdmerge.c
@@ -448,7 +448,7 @@
 /* Declarations for ordered dithering
  *
  * We use a 4x4 ordered dither array packed into 32 bits.  This array is
- * sufficent for dithering RGB888 to RGB565.
+ * sufficient for dithering RGB888 to RGB565.
  */
 
 #define DITHER_MASK       0x3