fix escape char problem in docstring

Use raw string literals

my_pyscf/mcscf/productstate.py

@@ -11,7 +11,7 @@
 
 # TODO: linkstr support
 class ProductStateFCISolver (StateAverageNMixFCISolver, lib.StreamObject):
-    '''Minimize the energy of a wave function of the form
+    r'''Minimize the energy of a wave function of the form
 
     |Psi> = A \prod_K |ci_K>
 
@@ -332,7 +332,7 @@ def state_average_fcisolver (solver, weights=(.5,.5), wfnsym=None):
     return state_average_mcscf (dummy, weights=weights, wfnsym=wfnsym).fcisolver
 
 class ImpureProductStateFCISolver (ProductStateFCISolver):
-    '''Minimize the energy of an impure state:
+    r'''Minimize the energy of an impure state:
 
     E = \sum_n1 w_n1 \sum_n2 w_n2 \sum_n3 w_n3 ... <n1n2n3...|H|n1n2n3...>