misc(spelling): fix discernable to discernible

[GaryJones]

See #2489.
See #2496.

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[GaryJones]

CLA signed.

[googlebot]

CLAs look good, thanks!

[paulirish]

thx!

my guess is #2496 went in while failureDescriptions was being introduced.

thanks for the contribution!

[GaryJones]

The Travis build appears to have timed out, rather than explicitly failed.