[epiphany] Update HACKING for meson

[Michael Catanzaro <mcatanzaro src gnome org>]

commit 6209ac2305c126a67323f60ec2898d5b166610b4
Author: Michael Catanzaro <mcatanzaro igalia com>
Date:   Tue Apr 17 20:53:49 2018 -0500

    Update HACKING for meson

 HACKING |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)
---
diff --git a/HACKING b/HACKING
index 77cebd6..30865f0 100644
--- a/HACKING
+++ b/HACKING
@@ -181,7 +181,7 @@ to the UI process.
 DEBUGGING
 =========
 
-To enable debugging use the configure option --enable-debug.
+To enable debugging use the configure option -Ddeveloper_mode=true.
 
 LOGGING
 =======