Documentation: fix reference to ifExists for interpret-trailers

The manual for "git interpret-trailers" mentioned a non-existing
literal `overwrite` for its config option `trailer.ifexists`.  The
correct name for that choice is `replace`.

Signed-off-by: Andreas Heiduk <asheiduk@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-interpret-trailers.txt

@@ -123,7 +123,7 @@ trailer.ifexists::
 	same <token> in the message.
 +
 The valid values for this option are: `addIfDifferentNeighbor` (this
-is the default), `addIfDifferent`, `add`, `overwrite` or `doNothing`.
+is the default), `addIfDifferent`, `add`, `replace` or `doNothing`.
 +
 With `addIfDifferentNeighbor`, a new trailer will be added only if no
 trailer with the same (<token>, <value>) pair is above or below the line