Fix exponent in docs (#102)

Author: raphaelsaavedra

docs/src/manual.md

@@ -35,7 +35,7 @@ For instance, for a particular distribution, one might want to ensure that the t
 parameter is positive: ``f \in \mathbb{R}^+``. The way to do this is to model ``\tilde{f} = \ln{f}``. 
 More generally, one can establish that ``\tilde{f} = h(f)``. We refer to this procedure as 
 **linking**. When a parameter is linked, the GAS recursion happens in the domain of ``\tilde{f}`` 
-and then one can recover the original parameter by ``f = \left(h\right)^-1(\tilde f)``. 
+and then one can recover the original parameter by ``f = \left(h\right)^{-1}(\tilde f)``. 
 We refer to this procedure as **unlinking**. The new recursion becomes:
 
 ```math