Fixed a bug in the cpp example and moved it to the examples directory.

docs/examples/tutorial/string/cpp_string.pyx

+# distutils: language = c++
+
+from libcpp.string cimport string
+
+def get_bytes():
+    py_bytes_object = b'hello world'
+    cdef string s = py_bytes_object
+
+    s.append('abc')
+    py_bytes_object = s
+    return py_bytes_object
+

docs/src/tutorial/strings.rst

@@ -365,18 +365,9 @@ C++ strings
 
 When wrapping a C++ library, strings will usually come in the form of
 the :c:type:`std::string` class.  As with C strings, Python byte strings
-automatically coerce from and to C++ strings::
+automatically coerce from and to C++ strings:
 
-    # distutils: language = c++
-
-    from libcpp.string cimport string
-
-    cdef string s = py_bytes_object
-    try:
-        s.append('abc')
-        py_bytes_object = s
-    finally:
-        del s
+.. literalinclude:: ../../examples/tutorial/string/cpp_string.pyx
 
 The memory management situation is different than in C because the
 creation of a C++ string makes an independent copy of the string