i18n: fix typos found during l10n for git 2.22.0

Fix two typos introduced by the following commits:

+ 31fba9d3b4 (diff-parseopt: convert --[src|dst]-prefix, 2019-03-24)
+ ed8b4132c8 (remote-curl: mark all error messages for translation,
  2019-03-05)

Signed-off-by: Jiang Xin <worldhello.net@gmail.com>
Acked-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/diff.c b/diff.c
index 2068b1c..a654d46 100644
--- a/diff.c
+++ b/diff.c
@@ -5354,7 +5354,7 @@ static void prep_parse_options(struct diff_options *options)
 			     N_("show the given source prefix instead of \"a/\""),
 			     PARSE_OPT_NONEG),
 		OPT_STRING_F(0, "dst-prefix", &options->b_prefix, N_("<prefix>"),
-			     N_("show the given source prefix instead of \"b/\""),
+			     N_("show the given destination prefix instead of \"b/\""),
 			     PARSE_OPT_NONEG),
 		OPT_CALLBACK_F(0, "line-prefix", options, N_("<prefix>"),
 			       N_("prepend an additional prefix to every line of output"),