Minor improvements to docs examples.

docs/examples/tutorial/string/for_bytes.pyx

-cdef bytes bytes_string = b'hello world'
+cdef bytes bytes_string = b"hello to A bytes' world"
 
 cdef char c
 for c in bytes_string:

docs/examples/tutorial/string/for_char.pyx

-cdef char* c_string = "Hello world"
+cdef char* c_string = "Hello to A C-string's world"
 
 cdef char c
 for c in c_string[:11]:

docs/examples/tutorial/string/if_char_in.pyx

@@ -2,4 +2,4 @@ cpdef void is_in(Py_UCS4 uchar_val):
     if uchar_val in u'abcABCxY':
         print("The character is in the string.")
     else:
-        print("The character isn't in the string")
+        print("The character is not in the string")