Add missing quote in circuits.ipynb (#4320)

Adds a missing quote in the Circuits notebook that causes incorrect display of the doc:

![TempCirqOperations](https://user-images.githubusercontent.com/83899250/125640450-7fbcc35c-c8cb-4c03-af46-37f2ab235c11.png)

docs/circuits.ipynb

@@ -205,7 +205,7 @@
    "source": [
     "The above is not the only way one can construct moments, nor even the typical method, but illustrates that a `Moment` is just a collection of operations on disjoint sets of qubits.\n",
     "\n",
-    "Finally, at the top level a `Circuit` is an ordered series of `Moment` objects. The first `Moment` in this series contains the first `Operations that will be applied. Here, for example, is a simple circuit made up of two moments:"
+    "Finally, at the top level a `Circuit` is an ordered series of `Moment` objects. The first `Moment` in this series contains the first `Operations` that will be applied. Here, for example, is a simple circuit made up of two moments:"
    ]
   },
   {