docs: Remove error in requestPermissions doc

If an app has already been granted a permission and it requests that
permission again, the user shouldn't see a system dialog. Update the
documentation to reflect this intended behavior.

Test: m ds-docs-java

Bug: 157530031

Exempt-From-Owner-Approval: Docs-only change
Change-Id: I7096e17480831324dee72f4093cdaa064e3b9165