typo fix

Author: The-Ray-Man

backend/src/api/utils.rs

@@ -719,7 +719,7 @@ fn check_not_free_condition(formulas: Vec<&Formula>, var: &String) -> BackendRes
             let captured = get_all_captures(f)?;
             if !captured.contains(var) {
                 return Err(BackendError::BadRequest(format!(
-                    "In {} the variable {} could occure freely",
+                    "In {} the variable {} could occur freely",
                     f,
                     var
                 )));