Fix incorrect command reminder after installs

[nmagee]

Issue

When installing new packages using pipenv install <package> the following reminder may be returned when the Pipfile is out of date:

Pipfile.lock (<hash>) out of date: run `pipfile lock` to update to (<new-hash>)...

However, no such command pipfile lock exists. The next returned line proceeds to run the correct command for the user:

Running $ pipenv lock then $ pipenv sync.

So it appears the author actually meant the reminder message to read pipenv lock since that is what is subsequently run.

Fix

Updated line of pipenv/routines/install.py to return correct command. See here

[oz123]

Looks OK. Please add a news file under news/

[nmagee]

@oz123 this is all set now. Thanks!