fix typo in MIGRATING.md

[jrectenwald]

No description provided.

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[jrectenwald]

I signed the CLA

[googlebot]

CLAs look good, thanks!

[coveralls]

Coverage remained the same at 96.076% when pulling 1923fbf on jrectenwald:patch-1 into 6bf3f7d on google:master.

[coveralls]

Coverage remained the same at 96.076% when pulling 1923fbf on jrectenwald:patch-1 into 6bf3f7d on google:master.