MDL-79264 tiny_equation: Correct debounce method

The debounced method was being called immediately instead of being
debounced.

lib/editor/tiny/plugins/equation/amd/src/ui.js

@@ -66,6 +66,7 @@ const displayDialogue = async(editor) => {
     currentForm = root.querySelector(Selectors.elements.form);
 
     const contextId = getContextId(editor);
+    const debouncedPreviewUpdater = debounce(() => updatePreview(getContextId(editor)), 500);
 
     $root.on(ModalEvents.hidden, () => {
         modalPromises.destroy();
@@ -95,14 +96,14 @@ const displayDialogue = async(editor) => {
             modalPromises.destroy();
         }
         if (textArea) {
-            debounce(updatePreview(contextId), 500);
+            debouncedPreviewUpdater();
         }
     });
 
     root.addEventListener('keyup', (e) => {
         const textArea = e.target.closest(Selectors.elements.equationTextArea);
         if (textArea) {
-            debounce(updatePreview(contextId), 500);
+            debouncedPreviewUpdater();
         }
     });