adapt doctest to have a different arcsech() expression

Author: rwst

src/sage/symbolic/expression.pyx

@@ -3316,8 +3316,8 @@ cdef class Expression(CommutativeRingElement):
             1/x
             sage: ~SR(3)
             1/3
-            sage: v1=var('v1'); a = (2*erf(2*v1*arcsech(0))/v1); ~a
-            1/2*v1/erf(2*v1*arcsech(0))
+            sage: v1=var('v1'); a = (2*erf(2*v1*arcsech(1/2))/v1); ~a
+            1/2*v1/erf(2*v1*arcsech(1/2))
         """
         return 1/self