Fix disk_allocation_ratio on filter_scheduler.rst

There is an option "disk_allocation_ratio" in DiskFilter instead of
a typo "disk_allocation_ration".
This patch fixes the typo on filter_scheduler.rst.

Change-Id: Iad4951fbdca97eccb956c42270bfc170fc7c7dac
Merge "Fix disk_allocation_ratio on filter_scheduler.rst"