Fixed typo in JSON

Somehow an extra character creeped in, making the notebook
invalid JSON, and thus unable to be opened.

00-Preface.ipynb

@@ -18,7 +18,7 @@
    "cell_type": "raw",
    "metadata": {},
    "source": [
-    "\\addcontentsline{toc}{chapter}{Preface}"c
+    "\\addcontentsline{toc}{chapter}{Preface}"
    ]
   },
   {