[tracker/sam/HACKING-update] HACKING.md: Update for change in functional-tests

[Sam Thursfield <sthursfield src gnome org>]

commit 9c8b71015e5d59405f01652c9fcf893665889be0
Author: Sam Thursfield <sam afuera me uk>
Date:   Wed Aug 18 15:58:43 2021 +0200

    HACKING.md: Update for change in functional-tests

 HACKING.md | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
---
diff --git a/HACKING.md b/HACKING.md
index 36efbd1f5..8330dbc03 100644
--- a/HACKING.md
+++ b/HACKING.md
@@ -92,5 +92,5 @@ commandline is controlled by the D-Bus .service.in files stored in
     Exec=/usr/bin/valgrind @abs_top_builddir@/src/tracker-store/tracker-store
 
 By default the run-uninstalled script and the functional-tests will only show
-output from Tracker code. For the functional-tests, set TRACKER_TESTS_VERBOSE=1
+output from Tracker code. For the functional-tests, set TRACKER_DEBUG=tests
 to see output from Valgrind. For tracker-sandbox use the `--debug-dbus` option.