docs: Change "ChromeOS" to "Chrome OS".

tools/mb/docs/design_spec.md

 # The MB (Meta-Build wrapper) design spec
 
 [TOC]
 
 ## Intro
 
 MB is intended to address two major aspects of the GYP -> GN transition
 for Chromium:
 
 1. "bot toggling" - make it so that we can easily flip a given bot
@@ skipping 393 matching lines @@
 This design allows us to determine easily all of the different sets
 of flags that we need to support, but *not* which flags are used on which
 host/target combinations.
 
 It may be that we should really track the latter. Doing so is just a
 config file change, however.
 
 ### Non-goals
 
 * MB is not intended to replace direct invocation of GN or GYP for
-  complicated build scenarios (aka ChromeOS), where multiple flags need
+  complicated build scenarios (a.k.a. Chrome OS), where multiple flags need
   to be set to user-defined paths for specific toolchains (e.g., where
-  ChromeOS needs to specify specific board types and compilers).
+  Chrome OS needs to specify specific board types and compilers).
 
 * MB is not intended at this time to be something developers use frequently,
   or to add a lot of features to. We hope to be able to get rid of it once
   the GYP->GN migration is done, and so we should not add things for
   developers that can't easily be added to GN itself.
 
 * MB is not intended to replace the
   [CR tool](https://code.google.com/p/chromium/wiki/CRUserManual). Not
   only is it only intended to replace the gyp\_chromium part of `'gclient
   runhooks'`, it is not really meant as a developer-facing tool.