[epiphany] Improve instructions for running valgrind

[Michael Catanzaro <mcatanzaro src gnome org>]

commit e95c0dde94df32d26386f55025f1b83eabe93e45
Author: Michael Catanzaro <mcatanzaro gnome org>
Date:   Mon Dec 2 19:43:36 2019 +0000

    Improve instructions for running valgrind

 HACKING.md | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
---
diff --git a/HACKING.md b/HACKING.md
index f8e7c0f61..b634a003c 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 epiphany
+WEBKIT_FORCE_SANDBOX=0 GIGACAGE_ENABLED=0 G_SLICE=always-malloc epiphany
 ```