Update 5-dataobject-relationship-management.md

typos
Merge pull request #3682 from bendubuisson/patch-1

Update 5-dataobject-relationship-management.md
FIX: Manifest flushing

Fixes silverstripe/silverstripe-framework#2325
Fixes silverstripe/silverstripe-framework#3093

Static manifest was not being flushed during a dev/build on some environments (without ?flush in URL) and template manifest was never being flushed during a dev/build.