fix script to delete old cloud formation stacks (#8)

## Summary

- Routine Change

### Details

- fix script to delete old cloud formation stacks

Author: anthony-nhs

.github/scripts/delete_stacks.sh

@@ -7,7 +7,7 @@ mapfile -t ACTIVE_STACKS_ARRAY <<< "$ACTIVE_STACKS"
 for i in "${ACTIVE_STACKS_ARRAY[@]}"
 do 
   echo "Checking if stack $i has open pull request"
-  PULL_REQUEST=${i//pr-/}
+  PULL_REQUEST=${i//fhir-validator-pr-/}
   PULL_REQUEST=${PULL_REQUEST//sandbox-/}
   echo "Checking pull request id ${PULL_REQUEST}"
   URL="https://api.github.com/repos/NHSDigital/eps-FHIR-validator-lambda/pulls/${PULL_REQUEST}"