docs: Do not remove too much from of the dumped online help

Use the len of the end and not the filename.
diff --git a/openbsc/contrib/dump_all_docs.py b/openbsc/contrib/dump_all_docs.py
index 314f60b..2903f80 100755
--- a/openbsc/contrib/dump_all_docs.py
+++ b/openbsc/contrib/dump_all_docs.py
@@ -26,7 +26,7 @@
 
 	# Now write everything until the end to the file
 	out = open(filename, 'w')
-	out.write(xml[18:len(filename)*-1])
+	out.write(xml[18:len(end)*-1])
 	out.close()