Fix abs code sample in doc/site/modules/core/num

-123.abs returns -123 because the dot has higher precedence so it's
 really -(123.abs).

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**