gis/grass: Replaced backtick cmdsub.

Signed-off-by: B. Watson <urchlay@slackware.uk>

Signed-off-by: Willy Sudiarto Raharjo <willysr@slackbuilds.org>

gis/grass/grass.SlackBuild

@@ -26,7 +26,7 @@ cd $(dirname $0) ; CWD=$(pwd)
 
 PRGNAM=grass
 VERSION=${VERSION:-7.8.0}
-SHRTVER=`echo $VERSION | sed "s/\([0-9]*\).\([0-9]*\)..*/\1\2/"`
+SHRTVER=$( echo $VERSION | sed "s/\([0-9]*\).\([0-9]*\)..*/\1\2/" )
 BUILD=${BUILD:-1}
 TAG=${TAG:-_SBo}
 PKGTYPE=${PKGTYPE:-tgz}