fix typo

src/borg/archiver.py

@@ -2158,7 +2158,7 @@ class Archiver:
             This pattern style is (only) useful to match full paths.
             This is kind of a pseudo pattern as it can not have any variable or
             unspecified parts - the full path must be given. `pf:root/file.ext` matches
-            `root/file.txt` only. A leading path separator is always removed.
+            `root/file.ext` only. A leading path separator is always removed.
 
             Implementation note: this is implemented via very time-efficient O(1)
             hashtable lookups (this means you can have huge amounts of such patterns