samples: add update intent code sample

[galz10]

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Fixes #<issue_number_goes_here> ☕️

[snippet-bot]

Here is the summary of changes.

You are about to add 1 region tag.

• samples/snippets/src/main/java/com/example/dialogflow/UpdateIntent.java:19, tag dialogflow_es_update_intent

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[telpirion]

Looks really good!