Fix wrong namespaceSelector configuration in snippet in document (#3054)

The snippet[1] was not aligning with the schema defined in CRD.

[1] https://external-secrets.io/latest/guides/security-best-practices/

Signed-off-by: kyasbal <kyasbal1994@gmail.com>

docs/guides/security-best-practices.md

@@ -24,8 +24,9 @@ metadata:
   name: fake
 spec:
   conditions:
-    namespaceSelector:
-      app: frontend
+    - namespaceSelector:
+        matchLabels:
+          app: frontend
 ```
 
 ### 3. Selectively Disable Reconciliation of Cluster-Wide Resources