Clarify lecture note of command line environment

[zjwu0522]

First, thanks for this wonderful lecture!

I made some clarifications on Command-line Environment's note.

I'm also doing this commit for the exercise in Version Control Lecture.

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This PR was originally from a fork: Chuhao95/missing-semester (branch: clarify-lecture-note-of-command-line-environment)

[zjwu0522]

Original author: @JJGO

Thanks for the contribution! Some comments:

• ln -s Path/to/your/dotfiles ~/.dotfiles is not what we are referring here. What we are really aiming for is something like ln -s ~/.dotfiles/bash/bashrc ~/.bashrc or with relative symlinks cd $HOME && ln -s ./.dotfiles/bash/bashrc .bashrc.
• Edit at Line 495 I'm not a fan of, I like the previous version better. There's also a typo (Creat)
• The numbered list being all ones is a feature, not a bug. See the Markdown syntax reference. This is more flexible as entries can be added/removed more conveniently