Fix Small Typos in intro.md, Lecture Numbering, and Notations

[HumphreyYang]

Hi @mmcky,

This is a quick fix for a typo in intro.md.

Please merge if it is a typo.

Many thanks in advance.

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[HumphreyYang]

I also find one lecture is numbered:

I have added a numbered tag in the _toc.yml.

[HumphreyYang]

Hi Matt,

graphviz_2.42.2 seems to cause some troubles:

E: Failed to fetch http://azure.archive.ubuntu.com/ubuntu/pool/universe/g/graphviz/graphviz_2.42.2-6_amd64.deb  Could not connect to azure.archive.ubuntu.com:80 (20.106.104.242), connection timed out [IP: 20.106.104.242 80]
E: Failed to fetch http://azure.archive.ubuntu.com/ubuntu/pool/universe/g/gts/libgts-bin_0.7.6%2bdarcs121130-5_amd64.deb  Unable to connect to azure.archive.ubuntu.com:http: [IP: 20.106.104.242 80]
E: Unable to fetch some archives, maybe run apt-get update or try with --fix-missing?

[mmcky]

thanks @HumphreyYang