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
1 file changed