Fixed error

doc/source/plugin_development.rst

@@ -105,13 +105,13 @@ The class must be derived from ``BaseDataSource``), and reimplement
 the appropriate methods:
 
 - ``run()``: where the actual computation takes place, given the
- configuration options specified in the model (which is received as an
- argument). It is strongly advised that the ``run()`` method is stateless.
+  configuration options specified in the model (which is received as an
+  argument). It is strongly advised that the ``run()`` method is stateless.
 - ``slots()``: must return a 2-tuple of tuples. Each tuple contains instances
- of the ``Slot`` class. Slots are the input and output entities of the
- data source. Given that this information depends on the
- configuration options, ``slots()`` accepts the model and must return the
- appropriate values according to the model options.
+  of the ``Slot`` class. Slots are the input and output entities of the
+  data source. Given that this information depends on the
+  configuration options, ``slots()`` accepts the model and must return the
+  appropriate values according to the model options.
 
 The MCO class
 ^^^^^^^^^^^^^