doc/howto_release.txt: be sure to clean garbage (trac https://trac.osgeo.org/grass/ticket/2845)

git-svn-id: https://svn.osgeo.org/grass/grass/branches/releasebranch_7_0@67512 15284696-431f-4ddb-bdfa-cd5b030d7da7

doc/howto_release.txt

@@ -45,6 +45,7 @@ Contents
        find . -name '*.rej'  | xargs rm
        find . -name '*.o'    | xargs rm
        find . -name '*.pyc'  | xargs rm
+       rm -f gui/wxpython/menustrings.py gui/wxpython/build_ext.pyc gui/wxpython/xml/menudata.xml gui/wxpython/xml/module_tree_menudata.xml
        chmod -R a+r *
 
        svn status