Update MarketplaceService.yaml (#899)

Fixed DisplaySubscriptionPeriod type being incorrectly set to a number
instead of string

## Changes

<!-- Please summarize your changes. -->

Just a type correction on the docs, I tried referencing this and got
back `/month` instead of a number.

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reports, etc.). -->

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Co-authored-by: Andrew Etter <[email protected]>

content/en-us/reference/engine/classes/MarketplaceService.yaml

@@ -598,7 +598,7 @@ methods:
         </tr>
       	<tr>
       	  <td><code>DisplaySubscriptionPeriod</code></td>
-      	  <td>number</td>
+      	  <td>string</td>
           <td>Localized subscription period text for display (for example, <code>/month</code>). Can be used together with <code>DisplayPrice</code>.</td>
         </tr>
       	<tr>