10829 zpool-features.5 add missing .RE

illumos/illumos-gate@1d775d5906fd85b9ab1ac2251af8d38b5567d9a1
https://github.com/illumos/illumos-gate/commit/1d775d5906fd85b9ab1ac2251af8d38b5567d9a1

https://www.illumos.org/issues/10829

zpool-features.5 is missing .RE before allocation_classes.

Author: Toomas Soome <tsoome@me.com>