Fix typo for area chart embeds

What does this MR do?

We can embed metric charts in markdown for alerts from externally-managed prometheus instances. Unfortunately, there's a typo in the dashboard code which assigns a chart type which does not exist!

This fixes that typo! line-graph (bad!) -> area-chart (good!)

Embeds still have worked up until this point because line-graph isn't a valid option and the default chart is an area-chart. So we've just defaulted to the correct behavior instead of choosing it.

This came up in discussions for: #217769 (comment 394014737)

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry - there is not a change in logic/behavior from a user perspective
• [-] Documentation (if required)
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Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

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