When the HTML output dir is specified using --dir=~/foo, the tilde

needs to be expanded by mkhowto, since the shell won't touch it except
at the start of the complete argument.

Doc/tools/mkhowto

@@ -205,7 +205,7 @@ class Options:
             elif opt == "--dir":
                 if os.sep == "\\":
                     arg = re.sub("/", "\\", arg)
-                self.builddir = arg
+                self.builddir = os.path.expanduser(arg)
             elif opt == "--paper":
                 self.paper = arg
             elif opt == "--dvips-safe":