Fix typos in the documentation for AutoBuffer. (#8197)

* Allocate 1000 floats to match the documentation

Fix the documentation of `AutoBuffer`. By default, the following code
```.cpp
cv::AutoBuffer<float> m;
```` 
allocates only 264 floats. But the comment in the demonstration code says it allocates 1000 floats, which is 
not correct.

* fix typo in the comment.