Update single-spa-html docs to reflect promise support (#377)

Reflect support for the single-spa-html template function returning a promise.

single-spa/single-spa-html#15

website/versioned_docs/version-5.x/ecosystem-html-web-components.md

@@ -55,6 +55,6 @@ singleSpa.registerApplication({
 
 ## API / Options
 single-spa-html is called with an object that has the following properties:
-- `template` (required): An HTML string or a function that returns a string. The function will be called with the single-spa custom props. The returned string is injected into the DOM during the single-spa mount lifecycle.
+- `template` (required): An HTML string or a function that returns a string or promise that resolves a string. The function will be called with the single-spa custom props. The returned string is injected into the DOM during the single-spa mount lifecycle.
 - `domElementGetter` (optional): A function that is given the single-spa props and returns the dom element container into which the HTML will be injected. If omitted,
   a default implementation is provided that wraps the template in a `<div>` that is appended to `document.body`.