Update clutter-free-headings.css

Adding ';' for the consistency.

Author: Nick Flückiger

clutter-free-headings.css

@@ -1,5 +1,5 @@
 .markdown-preview-view h1, .cm-header-1 { font-size: 2.33rem; }
-.markdown-preview-view h2, .cm-header-2 { font-size: 1.83rem }
+.markdown-preview-view h2, .cm-header-2 { font-size: 1.83rem; }
 .markdown-preview-view h3, .cm-header-3 { font-size: 1.5rem; }
 .markdown-preview-view h4, .cm-header-4 { font-size: 1.33rem; }
 .markdown-preview-view h5, .cm-header-5 { font-size: 1.16rem; }