[L6] Correct misplaced parenthesis

Fix barbagroup#60

The parenthesis in In [6] were miscplaced.
@fidle caught the bug and opened an issue (barbagroup#60).
Thank you for reporting the error @fidle.

Also:

* Re-run notebook to get updated execution times.
* Update execution times in Markdown cells.
* Add info about machine used to re-run notebook.

lessons/06_Array_Operations_with_NumPy.ipynb

@@ -161,7 +161,7 @@
      "name": "stdout",
      "output_type": "stream",
      "text": [
-      "1 loop, best of 3: 1.94 s per loop\n"
+      "3.07 s ± 15.1 ms per loop (mean ± std. dev. of 7 runs, 1 loop each)\n"
      ]
     }
    ],
@@ -189,7 +189,7 @@
    "cell_type": "markdown",
    "metadata": {},
    "source": [
-    "With the \"raw\" Python code above, the best execution time achieved was 1.94 seconds.  Keep in mind that with these three nested loops, that the statements inside the **j** loop are being evaluated more than 650,000 times.   Let's compare that with the performance of the same code implemented with array operations:"
+    "With the \"raw\" Python code above, the mean execution time achieved was 3.07 seconds (on a MacBook Pro Mid 2012).  Keep in mind that with these three nested loops, that the statements inside the **j** loop are being evaluated more than 650,000 times.   Let's compare that with the performance of the same code implemented with array operations:"
    ]
   },
   {
@@ -201,7 +201,7 @@
      "name": "stdout",
      "output_type": "stream",
      "text": [
-      "100 loops, best of 3: 5.09 ms per loop\n"
+      "7.38 ms ± 105 µs per loop (mean ± std. dev. of 7 runs, 100 loops each)\n"
      ]
     }
    ],
@@ -212,7 +212,7 @@
     "\n",
     "for n in range(nt + 1): ##loop across number of time steps\n",
     "    un = u.copy()\n",
-    "    u[1:, 1:] = un[1:, 1:] - ((c * dt / dx * (un[1:, 1:] - un[1:, 0:-1])) -\n",
+    "    u[1:, 1:] = (un[1:, 1:] - (c * dt / dx * (un[1:, 1:] - un[1:, 0:-1])) -\n",
     "                              (c * dt / dy * (un[1:, 1:] - un[0:-1, 1:])))\n",
     "    u[0, :] = 1\n",
     "    u[-1, :] = 1\n",
@@ -224,7 +224,7 @@
    "cell_type": "markdown",
    "metadata": {},
    "source": [
-    "As you can see, the speed increase is substantial.  The same calculation goes from 1.94 seconds to 5.09 milliseconds.  2 seconds isn't a huge amount of time to wait, but these speed gains will increase exponentially with the size and complexity of the problem being evaluated.  "
+    "As you can see, the speed increase is substantial.  The same calculation goes from 3.07 seconds to 7.38 milliseconds.  3 seconds isn't a huge amount of time to wait, but these speed gains will increase exponentially with the size and complexity of the problem being evaluated.  "
    ]
   },
   {
@@ -353,7 +353,7 @@
    "name": "python",
    "nbconvert_exporter": "python",
    "pygments_lexer": "ipython3",
-   "version": "3.6.5"
+   "version": "3.6.8"
   }
  },
  "nbformat": 4,