Another shellcheck fix

Author: nyalldawson

scripts/spell_check/check_spelling.sh

@@ -56,7 +56,7 @@ while getopts ":rdl:" opt; do
       ;;
   esac
 done
-shift $(($OPTIND - 1))
+shift $((OPTIND - 1))
 
 if [ $# -ne 0 ]; then
   EXCLUDE=$(${GP}sed -e 's/\s*#.*$//' -e '/^\s*$/d' $AGIGNORE | tr '\n' '|' | ${GP}sed -e 's/|$//')