Doc: fix PyBytesWriter sample code (GH-160756)

(cherry picked from commit d824240)

Co-authored-by: Inada Naoki <songofacandy@gmail.com>

Author: miss-islington

Doc/c-api/bytes.rst

@@ -488,7 +488,7 @@ Variant using a pointer to compute the final size::
 
         char *str = PyBytesWriter_GetData(writer);
         memcpy(str, "abc", 3);
-        str += 1;
+        str += 3;
         return PyBytesWriter_FinishWithPointer(writer, str);
     }
 
@@ -537,7 +537,7 @@ Update ``_PyBytes_Resize()`` code
 
 Example of code using the soft deprecated ``_PyBytes_Resize()`` API::
 
-    void create_string(Py_ssize_t size)
+    PyObject* create_string(Py_ssize_t size)
     {
         PyObject *bytes = PyBytes_FromStringAndSize(NULL, size);
         if (bytes == NULL) {
@@ -555,7 +555,7 @@ Example of code using the soft deprecated ``_PyBytes_Resize()`` API::
 
 It can now be updated to::
 
-    void create_string(Py_ssize_t size)
+    PyObject* create_string(Py_ssize_t size)
     {
         PyBytesWriter *writer = PyBytesWriter_Create(size);
         if (writer == NULL) {