Update verification_summary.md

Author: AdamZWu

docs/spec/v1.0/verification_summary.md

@@ -254,15 +254,16 @@ WARNING: This is just for demonstration purposes.
 
 </div>
 
-The result of evaluating an artifact (or set of artifacts) against SLSA.
-SHOULD contain one value per SLSA track:
+The result of evaluating an artifact (or set of artifacts) against SLSA tracks.
+SHOULD be one of these values:
 
 -   SLSA BUILD track:
     -   `SLSA_BUILD_LEVEL_0`
     -   `SLSA_BUILD_LEVEL_1`
     -   `SLSA_BUILD_LEVEL_2`
     -   `SLSA_BUILD_LEVEL_3`
--   FAILED (Indicates policy evaluation failed, optional)
+-   General policy evaluation track:
+    -   `FAILED`
 
 Note that each SLSA level implies the levels below it in the same track.
 For example, `SLSA_BUILD_LEVEL_3` means (`SLSA_BUILD_LEVEL_1` +