fix: Handlebars sample

[rwoll]

When following this tutorial I encountered two issues:

The template from the documentation was rendering but missing substitutions for the customer's first and last name:

         # Customer Context
          First Name: 
          Last Name: 
          Age: 30
          Membership Status: Gold

In the above, which was the result of running the sample code, the Age (30) and Membership (Gold) variables worked.

After debugging, it looks like this was a result of the sample code with the input variables using different casing:

                { "customer", new
                    {
                        firstName = "John",
                        lastName = "Doe",
                        age = 30,
                        membership = "Gold",
                    }
                },

first_name v. firstName

The second issue was the for-loop did not properly work. The sample was rendering as:

      {% for item in history %}
      <message role="">
          
      </message>
      {% endfor %}

with the {% etc. literlly in the rendered output.

[learn-build-service-prod]

Learn Build status updates of commit 566d591:

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File	Status	Preview URL	Details
semantic-kernel/concepts/prompts/handlebars-prompt-templates.md	✅Succeeded

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