Update cellid.go

[bertday]

Hello!

This PR fixes a minor typo in documentation for CellID.Decode. Previously it said that the function is used to encode cell IDs.

Thank you and hope this is helpful!

[googlebot]

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[bertday]

@googlebot I signed it!

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[dsymonds]

Thanks. Please update the commit message so it's in the usual format, starting with the package name. See the repo history for the expected form.