Fix off-by-one error in aria-posinset (#8388)

Fix #8191

app/javascript/mastodon/components/intersection_observer_article.js

@@ -109,7 +109,7 @@ export default class IntersectionObserverArticle extends React.Component {
       return (
         <article
           ref={this.handleRef}
-          aria-posinset={index}
+          aria-posinset={index + 1}
           aria-setsize={listLength}
           style={{ height: `${this.height || cachedHeight}px`, opacity: 0, overflow: 'hidden' }}
           data-id={id}
@@ -121,7 +121,7 @@ export default class IntersectionObserverArticle extends React.Component {
     }
 
     return (
-      <article ref={this.handleRef} aria-posinset={index} aria-setsize={listLength} data-id={id} tabIndex='0'>
+      <article ref={this.handleRef} aria-posinset={index + 1} aria-setsize={listLength} data-id={id} tabIndex='0'>
         {children && React.cloneElement(children, { hidden: false })}
       </article>
     );