Work around readme-breaking change in GitHub Markdown

The #gh-light-mode-only / #gh-dark-mode-only images used to work, but
this stopped working in the past few months. I reported the breakage to
support, who suggested to use a <p> tag instead of <h1> [1], which does
work. Thanks support person!

[1]: test99500-test@53e71f6

readme.md

@@ -1,7 +1,7 @@
-<h1 align="center">
+<p align="center">
 <img src="app/logo-light-background.svg#gh-light-mode-only" alt="Musium" width="400">
 <img src="app/logo-dark-background.svg#gh-dark-mode-only" alt="Musium" width="400">
-</h1>
+</p>
 
 Musium is an album-centered music player. It is designed to run as a daemon on
 an always-on device connected to speakers. Playback can be controlled from