fix a grammatical error in ./Doc/c-api/concrete.rst, delete them

Author: F18-Maverick

Doc/c-api/concrete.rst

@@ -8,7 +8,7 @@ Concrete Objects Layer
 **********************
 
 The functions in this chapter are specific to certain Python object types.
-Passing them an object of the wrong type is not a good idea; if you receive an
+Passing an object of the wrong type is not a good idea; if you receive an
 object from a Python program and you are not sure that it has the right type,
 you must perform a type check first; for example, to check that an object is a
 dictionary, use :c:func:`PyDict_Check`.  The chapter is structured like the