pinwheel/backwards: Adjust reading-material sync through syncthing

Move `books` and into `reading-material`

hosts/backwards/modules/syncthing/default.nix

@@ -75,7 +75,7 @@ in
           };
 
           books = {
-            path = "/home/alex/sync/books";
+            path = "/home/alex/sync/reading-material/books";
             devices = [ "pinwheel" ];
             versioning = {
               type = "staggered";
@@ -85,8 +85,8 @@ in
             };
           };
 
-          "reading-material" = {
-            path = "/home/alex/sync/reading-material";
+          ttrpg = {
+            path = "/home/alex/sync/reading-material/ttrpg";
             devices = [
               "phone"
               "tablet"