nixos/homer: fix escape docstring

Changed files
+1 -1
nixos
modules
services
web-apps
+1 -1
nixos/modules/services/web-apps/homer.nix
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To add files such as icons or backgrounds, you can reference them in line such as
```nix
-
icon = "$\{./icon.png}";
+
icon = "''${./icon.png}";
```
This will add the file to the nix store upon build, referencing it by file path as expected by Homer.
'';