Update to install-composer.txt

[Jennifer Hodgdon]

I wanted to let you know that on this issue:
https://www.drupal.org/project/user_guide/issues/2980636
we made a change to the English source file install-composer.txt:

The instructions for using Composer to download the core software were updated, so that they would actually work (using the Composer project on GitHub).

Here is the commit for reference:
https://cgit.drupalcode.org/user_guide/commit/?id=c1b05e0

Suggested actions to respond to this change in each language would be either:
a) If the file hasn't been translated yet, copy the English source file to your language directory and commit it.
b) If the file has already been translated, open a new issue to revise the translation.