Update variables.md

docs/src/manual/variables.md

@@ -11,7 +11,7 @@ written as `op1 ~ op2`, defines the symbolic equality between two operations.
 ## Types
 
 `Sym`, `Term`, and `FnType` are from
-[SymbolicUtils.jl](https://juliasymbolics.github.io/SymbolicUtils.jl/api/). Note that in
+[SymbolicUtils.jl](https://symbolicutils.juliasymbolics.org/api/). Note that in
 Symbolics, we always use `Sym{Real}`, `Term{Real}`, and
 `FnType{Tuple{Any}, Real}`. To get the arguments of an `istree` object, use
 `arguments(t::Term)`, and to get the operation, use `operation(t::Term)`.