Outdated emacs key binding (C-c C-g) in tactics chapter.

[iensen]

https://github.com/leanprover/theorem_proving_in_lean4/blob/ce23823e0e33bd153095dc307eae0341da2d9915/tactics.md?plain=1#L122

C-c C-g seems to only work in lean3-mode: https://github.com/leanprover/lean3-mode lean4-mode uses C-c C-i instead: https://github.com/leanprover-community/lean4-mode

I would delete this line or talk about C-c C-i instead.