README.md : Update README.md

:Release Notes:
Add missing "build-webos" directory in prerequisites description
of README.md file

:Detailed Notes:

:Testing Performed:
No image changes and Build tested.

:QA Notes:
None.

:Issues Addressed:
[PLAT-79071] [OSE] README.md : Update README.md
Change-Id: Ide98c89138525eb774bfac0fe060f9f43efa6b28

README.md

@@ -25,6 +25,7 @@ Prerequisites
 =============
 Before you can build, you will need some tools.  If you try to build without them, bitbake will fail a sanity check and tell you what's missing, but not really how to get the missing pieces. On Ubuntu, you can force all of the missing pieces to be installed by entering:
 
+    $ cd build-webos
     $ sudo scripts/prerequisites.sh
 
 Also, the bitbake sanity check will issue a warning if you're not running under Ubuntu 14.04 64bit LTS.