[epiphany] Further improve valgrind instructions

[Michael Catanzaro <mcatanzaro src gnome org>]

commit 6569bad5139624a7cc0c206bab1e3c575f9f5c06
Author: Michael Catanzaro <mcatanzaro gnome org>
Date:   Sun Mar 22 10:19:11 2020 -0500

    Further improve valgrind instructions

 HACKING.md | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
---
diff --git a/HACKING.md b/HACKING.md
index d267fd34d..c68511b3a 100644
--- a/HACKING.md
+++ b/HACKING.md
@@ -215,5 +215,5 @@ To use valgrind, you must use environment variables to disable WebKit's sandbox
 and JavaScriptCore's Gigacage:
 
 ```
-WEBKIT_FORCE_SANDBOX=0 GIGACAGE_ENABLED=0 G_SLICE=always-malloc valgrind --track-origins=yes epiphany
+WEBKIT_FORCE_SANDBOX=0 GIGACAGE_ENABLED=0 G_SLICE=always-malloc valgrind --track-origins=yes 
--leak-check=full epiphany
 ```