Correct various typos in code comments

Found via `codespell -q 3`

Closes #263

jdcolor.c

@@ -592,7 +592,7 @@ ycck_cmyk_convert(j_decompress_ptr cinfo, JSAMPIMAGE input_buf,
 /* 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

jdmerge.c

@@ -448,7 +448,7 @@ h2v2_merged_upsample(j_decompress_ptr cinfo, JSAMPIMAGE input_buf,
 /* 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

jquant1.c

@@ -154,7 +154,7 @@ typedef struct {
    */
   boolean is_padded;            /* is the colorindex padded for odither? */
 
-  int Ncolors[MAX_Q_COMPS];     /* # of values alloced to each component */
+  int Ncolors[MAX_Q_COMPS];     /* # of values allocated to each component */
 
   /* Variables for ordered dithering */
   int row_index;                /* cur row's vertical index in dither matrix */

simd/arm64/jsimd_neon.S

@@ -63,7 +63,7 @@ _\fname:
     trn2            \x1\literal, \xi\literal, \x1\literal
 .endm
 
-/* Transpose elements of 2 differnet registers */
+/* Transpose elements of 2 different registers */
 .macro transpose x0, x1, xi, xilen, literal
     mov             \xi\xilen, \x0\xilen
     trn1            \x0\literal, \x0\literal, \x1\literal

wrjpgcom.c

@@ -580,7 +580,7 @@ main(int argc, char **argv)
     }
   }
   /* Duplicate the remainder of the source file.
-   * Note that any COM markers occuring after SOF will not be touched.
+   * Note that any COM markers occurring after SOF will not be touched.
    */
   write_marker(marker);
   copy_rest_of_file();