python: fix threads.__str__ automethod documentation

Indent the directive properly to attach it to Threads autoclass
documentation.

Fixes:

WARNING: don't know which module to import for autodocumenting
'__str__' (try placing a "module" or "currentmodule" directive in the
document, or giving an explicit module name)

bindings/python/docs/source/threads.rst

@@ -11,4 +11,4 @@
       iterator and broke list(Threads()). Use `len(list(msgs))`
       instead.
 
-.. automethod:: __str__
+   .. automethod:: __str__