writing_a_great_proposol.md: updates the Gsoc link

[Shiva-sankaran]

The old link was not valid anymore so it has been updated with this https://google.github.io/gsocguides/student/ Closes #642

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Checklist

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[TravisBuddy]

Travis tests have failed

Hey @Shiva-sankaran, Please read the following log in order to understand the failure reason. It'll be awesome if you fix what's wrong and commit the changes.

TravisBuddy Request Identifier: d0d2b110-381c-11ea-b5ea-ed59922799f5