Fix grammar mistake.

Author: Jerjou Cheng

appengine/ndb/cache/README.md

@@ -1,7 +1,7 @@
 ## App Engine Datastore NDB Cache Samples
 
 This contains snippets used in the NDB cache documentation, demonstrating
-various operation on ndb caches.
+various operations on ndb caches.
 
 <!-- auto-doc-link -->
 These samples are used on the following documentation page:

appengine/ndb/entities/README.md

@@ -1,7 +1,7 @@
 ## App Engine Datastore NDB Entities Samples
 
 This contains snippets used in the NDB entity documentation, demonstrating
-various operation on ndb entities.
+various operations on ndb entities.
 
 <!-- auto-doc-link -->
 These samples are used on the following documentation page:

appengine/ndb/properties/README.md

@@ -1,7 +1,7 @@
 ## App Engine Datastore NDB Properties Samples
 
 This contains snippets used in the NDB properties documentation, demonstrating
-various operation on ndb properties.
+various operations on ndb properties.
 
 <!-- auto-doc-link -->
 These samples are used on the following documentation page:

appengine/ndb/property_subclasses/README.md

@@ -1,7 +1,7 @@
 ## App Engine Datastore NDB Property Subclasses Samples
 
 This contains snippets used in the NDB property subclasses documentation,
-demonstrating various operation on ndb property subclasses.
+demonstrating various operations on ndb property subclasses.
 
 <!-- auto-doc-link -->
 These samples are used on the following documentation page: