docs: add a missing $ sign

Addresses https://github.com/systemd/systemd/pull/27283#pullrequestreview-1386816102.
Follow-up to 1a127aa02b.

diff --git a/docs/ENVIRONMENT.md b/docs/ENVIRONMENT.md
index 791ba8dd12942619686a857aa3b582a0cb3f4e2a..2ba222d3a627571aee6180c8fae937d08cebd89a 100644 (file)
--- a/docs/ENVIRONMENT.md
+++ b/docs/ENVIRONMENT.md
@@ -341,7 +341,7 @@ the journal instead of only when logging in debug mode.
 
 `systemd-sysusers`
 
-* `SOURCE_DATE_EPOCH` — if unset, the field of the date of last password change
+* `$SOURCE_DATE_EPOCH` — if unset, the field of the date of last password change
   in `/etc/shadow` will be the number of days from Jan 1, 1970 00:00 UTC until
   today. If `$SOURCE_DATE_EPOCH` is set to a valid UNIX epoch value in seconds,
   then the field will be the number of days until that time instead. This is to