Fix a typo in main.py

This patch fixes a typo "false" in main.py. The right token should
be "False".

Change-Id: Ic56f6f3290b30ce2dd56dbe1ce4c31821b981723
Reviewed-on: https://dawn-review.googlesource.com/1560
Commit-Queue: Jiawei Shao <jiawei.shao@intel.com>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>

generator/main.py

@@ -131,7 +131,7 @@ def link_object(obj, types):
                         arg.length = "constant"
                         arg.constant_length = 1
                     else:
-                        assert(false)
+                        assert(False)
                 elif a['length'] == 'strlen':
                     arg.length = 'strlen'
                 else: