add in missing word

README.md

@@ -35,7 +35,7 @@ If we wish to quickly add a new cell we can do so with the following steps:
 
 To delete a cell we once again should be in escape mode, and then press the `x` key.
 
-Of course, we'll want a way to undo our deletion.  From escape mode, you can press `z` to undo deletion of a cell.  Note that this is different from `cmd + z`.  Pressing `cmd + z` while in insert mode undoes any changes inside of a cell while, whether these changes be deletions or text insertions.  Pressing `z` from escape mode undoes the deletion a cell.
+Of course, we'll want a way to undo our deletion.  From escape mode, you can press `z` to undo deletion of a cell.  Note that this is different from `cmd + z`.  Pressing `cmd + z` while in insert mode undoes any changes inside of a cell while, whether these changes be deletions or text insertions.  Pressing `z` from escape mode undoes the deletion of a cell.
 
 Go to escape mode and press `x`.  This cell disappears!
 

index.ipynb

@@ -100,7 +100,7 @@
    "source": [
     "To delete a cell we once again should be in escape mode, and then press the `x` key.\n",
     "\n",
-    "Of course, we'll want a way to undo our deletion.  From escape mode, you can press `z` to undo deletion of a cell.  Note that this is different from `cmd + z`.  Pressing `cmd + z` while in insert mode undoes any changes inside of a cell while, whether these changes be deletions or text insertions.  Pressing `z` from escape mode undoes the deletion a cell.\n",
+    "Of course, we'll want a way to undo our deletion.  From escape mode, you can press `z` to undo deletion of a cell.  Note that this is different from `cmd + z`.  Pressing `cmd + z` while in insert mode undoes any changes inside of a cell while, whether these changes be deletions or text insertions.  Pressing `z` from escape mode undoes the deletion of a cell.\n",
     "\n",
     "Go to escape mode and press `x`.  This cell disappears!\n",
     "\n",