Fix incorrect tailwind migration (#30007)

Fixes https://github.com/go-gitea/gitea/issues/30005. Regression from
https://github.com/go-gitea/gitea/pull/29945.

There was only once instance of `tw-content-center` before that PR, so I
just ran below command and reverted that one instance.

```sh
perl -p -i -e 's#tw-content-center#tw-items-center#g' web_src/js/**/* templates/**/* models/**/* tests/**/*
```

web_src/js/components/RepoContributors.vue

@@ -303,7 +303,7 @@ export default {
 </script>
 <template>
   <div>
-    <div class="ui header tw-flex tw-content-center tw-justify-between">
+    <div class="ui header tw-flex tw-items-center tw-justify-between">
       <div>
         <relative-time
           v-if="xAxisMin > 0"