Fix reference to non-existant 'tests' target.

This target was previously folded into dm. Do not advertise it anymore.

BUG=None
TEST=None
R=borenet@google.com

Review URL: https://codereview.chromium.org/646843002
diff --git a/make.py b/make.py
index cb6c2a4..2bb00a2 100644
--- a/make.py
+++ b/make.py
@@ -8,7 +8,7 @@
 #

 # Some usage examples:

 #   make clean

-#   make tests

+#   make dm

 #   make bench BUILDTYPE=Release

 #   make gm GYP_DEFINES=skia_scalar=fixed BUILDTYPE=Release

 #   make all