corrected units on reinforced concrete example in example notebook

Author: jchkoch

docs/examples/geometry/section_library.ipynb

@@ -700,8 +700,8 @@
    "outputs": [],
    "source": [
     "ei = sec.get_eic(e_ref=concrete)\n",
-    "print(f\"I_eff = {ei[0]:.3e} mm6\")\n",
-    "print(f\"I_rec = {(300 * 600**3 / 12):.3e} mm6\")"
+    "print(f\"I_eff = {ei[0]:.3e} mm4\")\n",
+    "print(f\"I_rec = {(300 * 600**3 / 12):.3e} mm4\")"
    ]
   }
  ],