[docs] update forking instructions (#1080)

CONTRIBUTING.md

@@ -101,15 +101,15 @@ See [this blog post](https://blog.sgmansfield.com/2016/06/working-with-forks-in-
 In case this post disappears, here are the steps (slightly modified):
 
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-> Pull the original package from the canonical place with the standard go get command:
+> Fork the repository on GitHub or set up whatever other remote git repo you will be using. In this case, I would go to GitHub and fork the repository.
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-> `go get github.com/superseriousbusiness/gotosocial`
+> Now clone the upstream repo (not the fork):
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-> Fork the repository on GitHub or set up whatever other remote git repo you will be using. In this case, I would go to GitHub and fork the repository.
+> `mkdir -p ~/go/src/github.com/superseriousbusiness && git clone git@github.com:superseriousbusiness/gotosocial ~/go/src/github.com/superseriousbusiness/gotosocial`
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-> Navigate to the top level of the repository on your computer. Note that this might not be the specific package you’re using:
+> Navigate to the top level of the upstream repository on your computer:
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-> `cd $GOPATH/src/github.com/superseriousbusiness/gotosocial`
+> `cd ~/go/src/github.com/superseriousbusiness/gotosocial`
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 > Rename the current origin remote to upstream:
 >