fix deprecated example (#18721)

doc/astspec.txt

@@ -1082,7 +1082,7 @@ its entirety to see some of the complexities.
 Concrete syntax:
 
 .. code-block:: nim
-  type Obj[T] = object {.inheritable.}
+  type Obj[T] {.inheritable.} = object
     name: string
     case isFat: bool
     of true:
@@ -1094,10 +1094,18 @@ AST:
 
 .. code-block:: nim
   # ...
+  nnkPragmaExpr(
+    nnkIdent("Obj"),
+    nnkPragma(nnkIdent("inheritable"))
+  ),
+  nnkGenericParams(
+  nnkIdentDefs(
+    nnkIdent("T"),
+    nnkEmpty(),
+    nnkEmpty())
+  ),
   nnkObjectTy(
-    nnkPragma(
-      nnkIdent("inheritable")
-    ),
+    nnkEmpty(),
     nnkEmpty(),
     nnkRecList( # list of object parameters
       nnkIdentDefs(