Fix incorrect documentation about get method (apache#1114)

Author: alexott

manual/object_mapper/using/README.md

@@ -51,8 +51,8 @@ UUID userId = ...;
 User u = mapper.get(userId);
 ```
 
-`get`'s arguments must match the partition key components (number of
-arguments and their types).
+`get`'s arguments must match the primary key components (number of
+arguments, their types, and order).
 
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