Replace tabs with spaces in html docs

wkhtmltopdf is not expanding tabs within <pre> elements to 8 spaces as it
should. Workaround the problem by converting all tabs to an appropriate
number of spaces.

Doc/Manual/Varargs.html

@@ -636,7 +636,7 @@ example. For example:
       PyObject *o = PyTuple_GetItem(varargs,i);
       if (!PyString_Check(o)) {
           PyErr_SetString(PyExc_ValueError,"Expected a string");
-	  free(argv);
+          free(argv);
           return NULL;
       }
       argv[i] = PyString_AsString(o);