Fix _updateConstraints missing protected/private classifier. (j…

…upyterlab#15066)

It was meant to be private - simple typo in a previous PR.
To avoid breakages in case if anyone has adopted it in a subclass
I am marking it as `protected`. If anyone was using it as a public
method this was an error - it should not be used like that.

packages/completer/src/widget.ts

@@ -72,7 +72,7 @@ export class Completer extends Widget {
   /**
    * Cache style constraints from CSS.
    */
-  _updateConstraints() {
+  protected _updateConstraints() {
     const tempNode = document.createElement('div');
     tempNode.classList.add(LIST_CLASS);
     tempNode.style.visibility = 'hidden';