Update capped-collections.txt

Typo in a phrase 'use cases' was 'uses cases'.

Signed-off-by: Sam Kleinman <samk@10gen.com>
DOCS-2034: refactoring organization of a page
DOCS-2034: adding cross reference
DOCS-2038: linking expire data tutorial to collMod index flag
minor: whitespace
DOCS-2034: additional structure
DOCS-2038: adding additional links
minor: typo/build fix