Fix signed-plugins CI grep pattern for H5PLint.c

The "Verify signature verification code exists" step used:
  grep "Verify signature" src/H5PLint.c
but the actual comment in H5PLint.c is:
  /* Verify plugin signature before loading */

Update the grep to "Verify plugin signature" so it matches the
existing comment and the step no longer fails with exit code 1.

Author: brtnfld

.github/workflows/signed-plugins.yml

@@ -208,7 +208,7 @@ jobs:
     - name: Verify signature verification code exists
       run: |
         echo "Checking signature verification implementation..."
-        grep -A 3 "Verify signature" src/H5PLint.c
+        grep -A 3 "Verify plugin signature" src/H5PLint.c
         echo "All ranks verify independently in both serial and parallel modes"
 
     - name: Run comprehensive tests