fast-import: tighten M 040000 syntax

diff --git a/RelNotes b/RelNotes
index a479abbc60611a95d75a33b092a322cb5fa2832c..b942e499449d97aeb50c73ca2bdc1c6e6d528743 120000 (symlink)
--- a/RelNotes
+++ b/RelNotes
@@ -1 +1 @@
-Documentation/RelNotes-1.7.2.1.txt
\ No newline at end of file
+Documentation/RelNotes/1.7.4.txt
\ No newline at end of file