Update go installation command in README.md

[pjg11]

Installing using go get returns an error message:

$ go get -u -v github.com/cirocosta/asciinema-edit
go: go.mod file not found in current directory or any parent directory.
	'go get' is no longer supported outside a module.
	To build and install a command, use 'go install' with a version,
	like 'go install example.com/cmd@latest'
	For more information, see https://golang.org/doc/go-get-install-deprecation
	or run 'go help get' or 'go help install'.

The command is updated to use go install as suggested in the error message.

[guysoft]

Thanks that helped me!
Worth merging!