fix sample request and response

Author: rculpepper

content/vault/v1.21.x (rc)/content/api-docs/system/billing-pki-certificate-count.mdx

@@ -33,8 +33,8 @@ is provided.
 
 ```json
 {
-    "start_time": "2025-01-01T00:00:00Z",
-    "end_time": "2025-01-01T00:00:00Z"
+    "start_time": "2025-01-01",
+    "end_time": "2025-01-01"
 }
 ```
 
@@ -53,7 +53,7 @@ $ curl \
 {
     "months": [
         {
-            "timestamp": "2025-01-01T00:00:00Z",
+            "timestamp": "2025-01-01",
             "counts": {
                 "issued_certificates": 123
             }