Fix link to IdString in 2.14.13 release notes
Fix link to IdString in 2.14.13 release notes

The link was copied from the 2.15 release notes, where the change
was originally released, and links to the stable-2.15 version of
the code.

Change it to stable-2.14.

Change-Id: I021383ce65dcc00748e7ae519eb1d1b4be2bf547

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