docs: update bento_crd.rst (#501)

field bentoTag does not exist in specs

docs/source/concepts/bento_crd.rst

@@ -57,7 +57,7 @@ Example of a Bento
     name: my-bento
     namespace: my-namespace
   spec:
-    bentoTag: iris:1
+    tag: iris:1
     image: my-registry.com/my-repository/iris:1
     runners:
     - name: runner1