Fix mention of macOS XDG_CONFIG_HOME fallback.

Signed-off-by: Felix Fontein <felix@fontein.de>

Author: felixfontein

README.rst

@@ -231,7 +231,7 @@ configuration directory.
 - **macOS**
 
   - Looks for ``keys.txt`` in ``$XDG_CONFIG_HOME/sops/age/keys.txt``;
-  - Falls back to ``$HOME/Library/Application Support/sops/age/keys.txt``.
+  - Falls back to ``$HOME/Library/Application Support/sops/age/keys.txt`` if ``$XDG_CONFIG_HOME`` isn’t set.
 
 - **Windows**