Fix copy and paste error

[jonashaag]

'to_pickle' does not support the 'index' parameter

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}

[jonashaag]

Will not sign any CLA but hereby put the changes under public domain so you may merge anyways

[rlenain]

Hi @jonashaag. Thanks for your input, that's very valuable.

Unfortunately we require anyone who wishes to contribute to Surfboard to sign the CLA. I will not be able to merge this until you do.

If you prefer however, I can make this change myself.

[jonashaag]

Yes please

[rlenain]

This has been fixed now. Please pull the latest master version. Thanks very much!