minor change to 1 page layout

src/layouts/SinglePage.astro

···
 
 <Toolbar {backlink} />
 <div class="wrapper bwcontainer">
-  <header class="bwbox">
+  <header class="bwbox center">
     <slot name="header" />
   </header>
   <main class="bwbox">

src/pages/blog.astro

···
   }}
 >
   <SinglePage>
-    <h1 slot="header" class="center">Blog posts</h1>
+    <h1 slot="header">Blog posts</h1>
     {
       post.map((post) => (
         <BlogPost