docs: fix typo (backport #8272)

src/borg/archiver.py

@@ -3467,7 +3467,7 @@ class Archiver:
         For security reasons, this command only works on local repositories.
 
         To delete a config value entirely, use ``--delete``. To list the values
-        of the configuration file or the default values, use ``--list``.  To get and existing
+        of the configuration file or the default values, use ``--list``.  To get an existing
         key, pass only the key name. To set a key, pass both the key name and
         the new value. Keys can be specified in the format "section.name" or
         simply "name"; the section will default to "repository" and "cache" for