replace shortcut command "t" to "r" (probably a typo as the two keys are so close to each other!)

Author: Atlas7

examples/Notebook/Notebook Basics.ipynb

@@ -216,7 +216,7 @@
     "\n",
     "1. Basic navigation: `enter`, `shift-enter`, `up/k`, `down/j`\n",
     "2. Saving the notebook: `s`\n",
-    "3. Cell types: `y`, `m`, `1-6`, `t`\n",
+    "3. Cell types: `y`, `m`, `1-6`, `r`\n",
     "4. Cell creation: `a`, `b`\n",
     "5. Cell editing: `x`, `c`, `v`, `d`, `z`, `ctrl+shift+-`\n",
     "6. Kernel operations: `i`, `.`"