[3.10] Add missing 'is' to `cmath.log()` docstring (GH-102049) (#102280)

Fix missing 'is' in cmath.log() docstring.
(cherry picked from commit 71f614ef2a3d66213b9cae807cbbc1ed03741221)

Co-authored-by: Owain Davies <116417456+OTheDev@users.noreply.github.com>