Fix reference

docs/installation.txt

@@ -50,8 +50,6 @@ try to run the examples.
 
       $ git clone https://git.ligo.org/lscsoft/bilby.git
 
-.. _installing-python:
-
 Installing optional requirements
 ================================
 
@@ -67,6 +65,8 @@ gravitational-wave data analysis.
 2. By default, we only require the default sampler dynesty_ be installed. For
 instructions on installing other samplers, see :ref:`Installing samplers`.
 
+.. _installing-python:
+
 Installing python
 -----------------