fix docstring

Author: shashi

src/simplify.jl

@@ -11,7 +11,8 @@ simplify(x; expand=false,
 Simplify an expression (`x`) by applying `rewriter` until there are no changes.
 `expand=true` applies [`expand`](/api/#expand) in the beginning of each fixpoint iteration.
 
-Assumes denominators are not zero by default. Pass `nonzero_denominators=false` if otherwise.
+By default, simplify will assume denominators are not zero and allow cancellation in fractions.
+Pass `simplify_fractions=false` to prevent this.
 """
 function simplify(x;
                   expand=false,