Modified the description of the doc directory

In the latest version of Contributing to
DevStack manual.There are a few words to
say that "tools/build_docs.sh is used to
generate the HTML versions of the DevStack
scripts".But build_docs.sh is not there
since Newton version.So I delete it for
good.

Change-Id: I69f7aa23e1efd8f8a63aa79628e67378d524e173

Author: Chang Liu

HACKING.rst

@@ -47,8 +47,7 @@ The DevStack repo generally keeps all of the primary scripts at the root
 level.
 
 ``doc`` - Contains the Sphinx source for the documentation.
-``tools/build_docs.sh`` is used to generate the HTML versions of the
-DevStack scripts.  A complete doc build can be run with ``tox -edocs``.
+A complete doc build can be run with ``tox -edocs``.
 
 ``exercises`` - Contains the test scripts used to sanity-check and
 demonstrate some OpenStack functions. These scripts know how to exit