nested namespaces syntax

[jmeyers314]

I think https://github.com/lsst/templates/blob/master/project_templates/stack_package/example/src/Starter.cc#L25
is a syntax error. Should be namespace lsst { namespace example {

probably this line too: https://github.com/lsst/templates/blob/master/project_templates/stack_package/example_subpackage/src/Starter.cc#L25

[jonathansick]

Jira link: https://jira.lsstcorp.org/browse/DM-29922