Fix example in the documentation of promote_and_bradcast.

PiperOrigin-RevId: 357809698

Author: tfx-copybara

struct2tensor/expression_impl/promote_and_broadcast.py

@@ -94,9 +94,11 @@
 
 session: {
   event: {
+    nage: 20
     val: 7
   }
   event: {
+    nage: 20
     val: 8
     val: 9
   }