Fix doc and add child spec

[bighelmet7]

I am in the process of learning how the static_supervisor works, and I noticed that the significant property wasn't added to the convert_child. According to the Erlang documentation:

child_spec() = #{id => child_id(),             % mandatory
                 start => mfargs(),            % mandatory
                 restart => restart(),         % optional
                 significant => significant(), % optional
                 shutdown => shutdown(),       % optional
                 type => worker(),             % optional
                 modules => modules()}         % optional

We should be able to pass the ChildBuilder.significant to the convert_child which then will be passed to the supervisor:start_link.

Example of how the significant spec gets passed to running worker_child

    offender: [{pid,<0.864.0>},
               {id,<<"worker">>},
               {mfargs,{erlang,apply,
                               [#Fun<gleam@otp@static_supervisor.0.87899615>,
                                []]}},
               {restart_type,temporary},
               {significant,true},
               {shutdown,5000},
               {child_type,worker}]

[lpil]

Thank you!!