Fix message header mapping docs to show underscores

[josephearl]

Description

This fixes the docs for the JMS component to show that e.g. - is converted to _HYPHEN_ and not HYPHEN (in italics) as it was showing

Before:

After:

https://github.com/josephearl/camel/blob/main/components/camel-jms/src/main/docs/jms-component.adoc#message-header-mapping

[github-actions]

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