Clarify Array Slicing Syntax from 'array[start:stop[:step]]' to 'array[start:stop:step]' (#465)

* Clarified Array Slicing Syntax

* Update numpy-basics.ipynb

Thank you for the feedback. I appreciate the concern about the notation implying that the second colon and the step parameter are mandatory. 

I propose we adopt 'array[start:stop:step]' as the primary notation in examples but include a clear note in the text explaining that the step parameter is optional: "The step parameter is optional and can be omitted, in which case the slice uses a default step of 1."

* Resolved issue with wrong use of parenthesis ` `

Author: kazakhpunk

core/numpy/numpy-basics.ipynb

@@ -700,7 +700,7 @@
    "source": [
     "### Slices\n",
     "\n",
-    "Slicing syntax is written as `array[start:stop[:step]]`, where **all numbers are optional**.\n",
+    "Slicing syntax is written as `array[start:stop:step]`. Note that **all numbers are optional**. Importantly, the **step** parameter is optional and can be omitted, in which case the slice uses a default step of 1.\n",
     "- defaults: \n",
     "  - start = 0\n",
     "  - stop = len(dim)\n",