(maint) Rename let example instance variable

Previously, the test created an instance variable named example, for
which rspec created a method that memoized the instance variable. The
method shadowed the example method on RSpec::Core::ExampleGroup, which
can lead to surprising results. For example if you add an rspec before
hook:

RSpec.configure do |config|
  config.before :each do |test|
    ...
  end
end

Then test will be derived from RSpec::Core::ExampleGroup:

(rdb:1) test.is_a? RSpec::Core::ExampleGroup
true

But if you call test.example it will return the memoized variable,
instead of the RSpec::Core::Example:

(rdb:2) test.example.class
Puppet::Type::Ssh_authorized_key

This commit simply renames the instance variable/method.