disable shellcheck SC2048 on URL array

See: https://github.com/koalaman/shellcheck/wiki/SC2048

In the case of the UPDATE_PLATFORM_URLS variable, we actually want to
split on whitespace since the array should just contain URLs.

Disable the Shellcheck directive for this array.

Closes: #93

usr/lib/greenboot/check/wanted.d/01_update_platforms_check.sh

@@ -13,6 +13,7 @@ get_update_platform_urls() {
 }
 
 assert_update_platforms_are_responding() {
+    # shellcheck disable=SC2048
     for UPDATE_PLATFORM_URL in ${UPDATE_PLATFORM_URLS[*]}; do
         HTTP_STATUS=$(curl -o /dev/null -Isw '%{http_code}\n' "$UPDATE_PLATFORM_URL" || echo "Unreachable")
         if ! [[ $HTTP_STATUS == 2* ]] && ! [[ $HTTP_STATUS == 3* ]]; then