[mb] Fix typo in mb config

infra/mb/mb_config.pyl

 # Copyright 2016 The V8 project authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 {
   # This is a map of buildbot master names -> buildbot builder names ->
   # config names (where each config name is a key in the 'configs' dict,
   # below). MB uses this dict to look up which config to use for a given bot.
   # Bots are ordered by appearance on waterfall.
   'masters': {
@@ skipping 94 matching lines @@
       'gyp_defines': 'test_isolation_mode=prepare',
     },
 
     'valgrind': {
       # TODO(machenbach): Add this to gn.
       'gn_args': 'has_valgrind=true',
       'gyp_defines': 'has_valgrind=1',
     },
 
     'v8_enable_slow_dchecks': {
-      'gn_args': v8_enable_slow_dchecks=true',
+      'gn_args': 'v8_enable_slow_dchecks=true',
       'gyp_defines': 'v8_enable_slow_dchecks=1',
     },
 
     'v8_optimized_debug': {
       # This is the default in gn for debug.
       'gyp_defines': 'v8_optimized_debug=1',
     },
 
     'v8_snapshot_custom': {
       # TODO(machenbach): Add this to gn.
@@ skipping 10 matching lines @@
       'gn_args': 'target_cpu="x64"',
       'gyp_defines': 'target_arch=x64',
     },
 
     'x86': {
       'gn_args': 'target_cpu="x86"',
       'gyp_defines': 'target_arch=ia32',
     },
   },
 }