fix syntax

Author: aarone

scripts/build-postgres-darwin.sh

@@ -29,7 +29,7 @@ if echo "$PG_VERSION" | grep -q '^9\.' && [ "$LITE_OPT" = true ] ; then
   echo "Lite option is supported only for PostgreSQL 10 or later!" && exit 1;
 fi
 
-export MACOSX_DEPLOYMENT_TARGET="$(sw_vers -productVersion)")
+export MACOSX_DEPLOYMENT_TARGET="$(sw_vers -productVersion)"
 
 ICU_ENABLED=$(echo "$PG_VERSION" | grep -qv '^9\.' && [ "$LITE_OPT" != true ] && echo true || echo false)