Update install documentation

[JaneX8]

Problem:

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'.

go install github.com/cirocosta/asciinema-edit
# go: 'go install' requires a version when current directory is not in a module
#        Try 'go install github.com/cirocosta/asciinema-edit@latest' to install the latest version

Solution:

go install github.com/cirocosta/asciinema-edit@latest