Correctly import from private task-instance

[mydea]

In my typescript app I noticed these warnings:

WARNING in ./node_modules/ember-concurrency/-task-instance.js 4:15-27
"export 'default' (imported as 'TaskInstance') was not found in './-private/task-instance'
@ ./assets/my-app.js
    
WARNING in ./node_modules/ember-concurrency/index.js 11:0-378
"export 'default' (reexported as 'TaskInstance') was not found in './-private/task-instance'
@ ./node_modules/ember-power-select/components/power-select.js
@ ./assets/my-app.js

So I went and looked, and it seems they were incorrectly importing the default instead of named export. I guess this isn't used anywhere, as otherwise it should have broken (?) somehow. I fixed the imports here.

[maxfierke]

nice! yeah, that export is just for consumers that might have been importing TaskInstance via a private module import (naughty, naughty) from 1.x, so not a huge deal, but good to fix!