Fix third-party system libraries list in ActiveStorage::Preview [ci skip]

activestorage/app/models/active_storage/preview.rb

@@ -23,8 +23,8 @@
 #
 # The built-in previewers rely on third-party system libraries:
 #
-#   * {ffmpeg}[https://www.ffmpeg.org]
-#   * {mupdf}[https://mupdf.com]
+# * {ffmpeg}[https://www.ffmpeg.org]
+# * {mupdf}[https://mupdf.com]
 #
 # These libraries are not provided by Rails. You must install them yourself to use the built-in previewers. Before you
 # install and use third-party software, make sure you understand the licensing implications of doing so.