Improve clarity of text for merge train position

What does this MR do?

The current text in the merge request widget when in item is in the merge train is ambiguous. We say the merge train is "added to the merge train at position x" when we mean that it is currently in the merge train at that position (not that it was added there). This is a simple change that makes the text no longer ambiguous.

Screenshots

N/A, just changes text from "Added to the merge train at position x" to "In the merge train at position x"

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

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

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team