Change 'their' to the 'multisig_acc' in the changevest action description in the golos control contract

[e-schepachev]

In this piece of text:

In case a list of the most rated witnesses is changed, their authorization will automatically be changed too.

Change "their authorization" to "multisig_acc authorization"

[anazarov79]

The word combination "their authorization" is replaced by "multisig_acc authorization".