kijiproject.github.com: DOCS-155. Correct schema-shell user guide page information about dropping default reader schemas.

issue: https://jira.kiji.org/browse/DOCS-155
review: https://review.kiji.org/r/1630
commit: https://github.com/kijiproject/kijiproject.github.com/commit/ac303e815489694c39d3aa4e6956cc64d74491ff