Fix documentation of ExtractionTurbineCHPBlock

The electrical output power is greater than the thermal one.
(Except for extreme differences in the efficiencies,
but this is also part of the inequation.)

Fixes #565

oemof/solph/components.py

@@ -1098,7 +1098,7 @@ class ExtractionTurbineCHPBlock(SimpleBlock):
             \frac{P_{el} + \dot Q_{th} \cdot \beta}
                  {\eta_{el,woExtr}} \\
             &
-            P_{el} \leq \dot Q_{th} \cdot
+            P_{el} \geq \dot Q_{th} \cdot
             \frac{\eta_{el,maxExtr}}
                  {\eta_{th,maxExtr}}