[Doc] Fix typo in cpp/installing when wheel is used (#111143)
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### <samp>🤖 Generated by Copilot at 34a15a6</samp>
Updated the cmake command in `docs/cpp/source/installing.rst` to use python3 and fix a documentation error.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/111143
Approved by: https://github.com/clee2000, https://github.com/kit1980
