a-plumbers-guide-to-git: fix a typo on the intro page
a-plumbers-guide-to-git: fix a typo on the intro page

h/t @chailey_ on Twitter

alexwlchan/alexwlchan.net,alexwlchan/alexwlchan.net,alexwlchan/alexwlchan.net,alexwlchan/alexwlchan.net,alexwlchan/alexwlchan.net