Fixed grammar: before H the article is a, not an.

src/sds.c

@@ -582,7 +582,7 @@ int is_hex_digit(char c) {
            (c >= 'A' && c <= 'F');
 }
 
-/* Helper function for sdssplitargs() that converts an hex digit into an
+/* Helper function for sdssplitargs() that converts a hex digit into an
  * integer from 0 to 15 */
 int hex_digit_to_int(char c) {
     switch(c) {