Fixed default string for intitals mathml in PN (#854)

## Summary of Changes
Fixed the default value for expression_mathml in the Initial struct for
PetriNets

### Related issues

Resolves ???

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Co-authored-by: Justin <[email protected]>

skema/skema-rs/mathml/src/acset.rs

@@ -202,7 +202,7 @@ pub struct Initial {
 }
 
 impl Default for Initial {
-    fn default() -> Self { Initial { target: "temp".to_string(), expression: "0".to_string(), expression_mathml: "<math><\\math>".to_string() } }
+    fn default() -> Self { Initial { target: "temp".to_string(), expression: "0".to_string(), expression_mathml: "<math></math>".to_string() } }
 }
 
 #[derive(