use singular for class names consistently

doc/manual/src/SUMMARY.md.in

@@ -17,9 +17,9 @@
   - [Upgrading Nix](installation/upgrading.md)
 - [Architecture](architecture/architecture.md)
   - [Store](architecture/store/store.md)
-    - [Store Objects](architecture/store/objects.md)
-    - [Store Paths](architecture/store/paths.md)
-    - [Derivations](architecture/store/drvs/drvs.md)
+    - [Store Object](architecture/store/objects.md)
+    - [Store Path](architecture/store/paths.md)
+    - [Derivation](architecture/store/drvs/drvs.md)
 - [Package Management](package-management/package-management.md)
   - [Basic Package Management](package-management/basic-package-mgmt.md)
   - [Profiles](package-management/profiles.md)

doc/manual/src/architecture/store/drvs/drvs.md

@@ -1,4 +1,4 @@
-# Derivations
+# Derivation
 
 Derivations are recipes to create store objects.
 

doc/manual/src/architecture/store/paths.md

@@ -1,4 +1,4 @@
-# Store Paths
+# Store Path
 
 A store path is a pair of a 20-byte digest and a name.