Fixing typo in SignedMessage docstring #505

Job	Run time
Python 3.7 on macos-13	2m 59s
Python 3.7 on Windows x86	2m 4s
Python 3.12 on macos-13	3m 29s
Python 3.8 on Windows x86	2m 57s
Linux docs	1m 20s
Python 3.9 on Windows x86	3m 21s
Python 3.12 (minimal build) on macos-13	2m 44s
Linux meta	1m 6s
Python 3.10 on Windows x86	2m 13s
Python 3.12 on macos-latest	1m 31s
Linux mypy	1m 5s
Python 3.11 on Windows x86	2m 9s
Python 3.12 (minimal build) on macos-latest	1m 26s
Linux pypy3	2m 44s
Python 3.12 on Windows x86	2m 8s
Linux pypy3	2m 55s
Python 3.7 on Windows x64	2m 22s
Linux py37	1m 41s
Python 3.8 on Windows x64	2m 23s
Linux py38	1m 42s
Python 3.9 on Windows x64	2m 1s
Linux py39	1m 40s
Python 3.10 on Windows x64	2m 28s
Linux py310	1m 42s
Python 3.11 on Windows x64	2m 23s
Linux py311	1m 40s
Python 3.12 on Windows x64	2m 31s
Linux py312	1m 51s
Linux py312 system	2m 35s
Linux py312 minimal	1m 41s
	1h 4m 51s

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