Bump latest alpha version in readme

I had to release a2 from a different branch because it doesn't want to include the new dependency version restrictions that come with Mopidy 4 compat. So this readme now points to a version that doesn't exist in this branch. Oops.

Author: kingosticks

README.rst

@@ -90,7 +90,7 @@ Installation
 
 Install by running::
 
-    sudo python3 -m pip install --break-system-packages Mopidy-Spotify==5.0.0a1
+    sudo python3 -m pip install --break-system-packages Mopidy-Spotify==5.0.0a2
 
 
 Configuration