Fix CBC mode encryption example in classic.rst #1007

Job	Run time
mypy	17s
python_no_gil	3m 21s
build_python_27 (ubuntu-20.04, yes)	2m 44s
build (3.6, yes, ubuntu-20.04)	2m 43s
build_python_27 (ubuntu-20.04, no)	2m 49s
build (3.6, no, ubuntu-20.04)	2m 41s
test_c_linux (0, x64)	48s
build_python_27 (windows-latest, yes)	4m 15s
build (3.7, yes, ubuntu-20.04)	2m 36s
test_c_windows (x64)	1m 6s
test_c_linux (0, x32)	58s
build_python_27 (windows-latest, no)	3m 54s
build (3.7, no, ubuntu-20.04)	2m 33s
test_c_windows (win32)	1m 1s
test_c_linux (1, x64)	57s
build (3.8, yes, ubuntu-20.04)	2m 31s
test_c_linux (1, x32)	1m 9s
build (3.8, no, ubuntu-20.04)	2m 32s
build (3.9, yes, ubuntu-20.04)	2m 25s
build (3.9, no, ubuntu-20.04)	2m 26s
build (3.10, yes, ubuntu-20.04)	2m 26s
build (3.10, no, ubuntu-20.04)	2m 26s
build (3.11, yes, ubuntu-20.04)	2m 4s
build (3.11, no, ubuntu-20.04)	2m 4s
build (3.12, yes, ubuntu-20.04)	2m 10s
build (3.12, no, ubuntu-20.04)	2m 9s
build (3.13-dev, yes, ubuntu-20.04)	2m 10s
build (3.13-dev, no, ubuntu-20.04)	2m 29s
build (3.12, yes, macos-12)	3m 8s
build (3.12, no, windows-latest)	3m 53s
build (3.12, yes, windows-latest)	4m 17s
build (pypy2.7, no, ubuntu-latest)	1m 33s
build (pypy3.9, no, ubuntu-latest)	1m 44s
	1h 16m 19s

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