README: use full file names for default.xml and default-notools.xml

Passing 'default' or 'default-notools' to repo init's -m parameter fails because
the manifests are actually called 'default.xml' and 'default-notools.xml'
respectively.

Signed-off-by: Aaron Plattner <aplattner@nvidia.com>
Signed-off-by: Stephen Warren <swarren@nvidia.com>