var_expand: Fix typo systemt => system

Author: cmouse

.github/actions/spelling/expect.txt

@@ -784,7 +784,6 @@ sysexits
 syslogd
 syslogging
 sysreport
-systemt
 tabindex
 TABLEGEN
 Taddressverification

docs/core/design/var_expand.md

@@ -8,7 +8,7 @@ dovecotlinks:
 # Variable expansion design
 
 Dovecot comes with powerful variable expansion system, which allows constructing reusable text templates.
-This has been upgraded since v2.3 to a more flexible systemt .
+This has been upgraded since v2.3 to a more flexible system.
 
 ## Syntax