Make docs/udpate_docs.sh executable

Including a misc clean-up in the script.

Change-Id: I7bda3ac52e15712356cc286caba08e1fd9db68e1

docs/update_docs.sh

@@ -24,11 +24,11 @@ cd docs
 make html
 
 cd ../out
-rm -rf gh-pages
 
 # If SITE_DIRECTORY is specified, it is assumed to be local evaluation, so we
 # will not try to update Git repository.
 if [[ -z ${SITE_DIRECTORY} ]] ; then
+  rm -rf gh-pages
   git clone --depth 1 https://github.com/google/shaka-packager -b gh-pages gh-pages
   cd gh-pages
   git rm -rf *