fix: add explicit constructor to Foo class and fix abs example syntax in docs

- Add explicit `construct new() {}` to Foo class in error-handling doc to clarify method dispatch behavior
- Fix `(-123).abs` syntax in num.markdown to use parentheses for unary minus precedence

doc/site/error-handling.markdown

@@ -49,7 +49,9 @@ not found" one. If you call a method on an object and its class (and all of its
 superclasses) don't define that method, there's nothing Wren can do:
 
     :::wren
-    class Foo {}
+    class Foo {
+      construct new() {}
+    }
 
     var foo = Foo.new()
     foo.someRandomMethod

doc/site/modules/core/num.markdown

@@ -20,7 +20,7 @@ The value of π.
 The absolute value of the number.
 
     :::wren
-    System.print(-123.abs) //> 123
+    System.print( (-123).abs ) //> 123
 
 ### **acos**