Update windows_powershell_script_block_with_malicious_string.yml

Author: nterl0k

detections/endpoint/windows_powershell_script_block_with_malicious_string.yml

@@ -47,7 +47,7 @@ rba:
     score: 70
   threat_objects:
   - field: signature_id
-    type: signature_id
+    type: signature
 tags:
   analytic_story: 
   - Malicious PowerShell
@@ -59,11 +59,6 @@ tags:
   - Splunk Enterprise
   - Splunk Enterprise Security
   - Splunk Cloud
-  required_fields: 
-  - ActivityID
-  - Computer
-  - EventCode
-  - ScriptBlockText
   security_domain: threat
 tests:
 - name: True Positive Test