[Docs] Install script - Improve installation guide

[shivabhusal]

If installation is prompted to be successful, yet nvm command returns nvm: command not found then you might need to restart your terminal instance. Try opening a new tab/window in your terminal and retry.

[ljharb]

which nvm will always return not found, because nvm is not a binary or an executable; it's a sourced shell function. See https://github.com/creationix/nvm#verify-installation

[ljharb]

That said, this change looks good :-)

("restart your terminal" is exactly what the install script outputs to the terminal, but people don't read things)

[shivabhusal]

@ljharb I updated the description. However, the which nvm command does not always return not found.

$ which nvm
nvm () {
	if [ $# -lt 1 ]
	then
		nvm --help
		return
	fi
	local COMMAND
	COMMAND="${1-}" 
	shift
	local VERSION
	local ADDITIONAL_PARAMETERS
	case $COMMAND in

        ...