Fix user docs.

tools/mb/docs/user_guide.md

 # The MB (Meta-Build wrapper) user guide
 
 [TOC]
 
 ## Introduction
 
 `mb` is a simple python wrapper around the GYP and GN meta-build tools to
 be used as part of the GYP->GN migration.
 
 It is intended to be used by bots to make it easier to manage the configuration
@@ skipping 201 matching lines @@
     'shared': {
       'gn_args': 'is_component_build=true',
       'gyp_defines': 'component=shared_library',
     },
     'trybot': {
       'gyp_defines': 'dcheck_always_on=1',
       'gn_args': 'dcheck_always_on=true',
     }
   }
 }
+```
 
 and you ran `mb gen -c linux_release_trybot //out/Release`, it would
 translate into a call to `gyp_chromium -G Release` with `GYP_DEFINES` set to
 `"use_goma=true dcheck_always_on=false dcheck_always_on=true"`.
 
 (From that you can see that mb is intentionally dumb and does not
 attempt to de-dup the flags, it lets gyp do that).
 
 ## Debugging MB
 
 By design, MB should be simple enough that very little can go wrong.
 
 The most obvious issue is that you might see different commands being
 run than you expect; running `'mb -v'` will print what it's doing and
 run the commands; `'mb -n'` will print what it will do but *not* run
 the commands.
 
 If you hit weirder things than that, add some print statements to the
 python script, send a question to gn-dev@chromium.org, or
 [file a bug](https://crbug.com/new) with the label
 'mb' and cc: dpranke@chromium.org.