fix missing backticks for plot_histogram() (Qiskit/qiskit-tutorials#1480

)

### Summary
The function plot_histogram() in the text should be `plot_histogram()`


### Details and comments

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Co-authored-by: Eric Arellano <ericarellano@me.com>

docs/tutorials/circuits/2_plotting_data_in_qiskit.ipynb

@@ -170,7 +170,7 @@
    "source": [
     "### Using the output from plot_histogram()\n",
     "\n",
-    "When using the plot_histogram() function it returns a `matplotlib.Figure` for the rendered visualization. Jupyter notebooks understand this return type and render it for us in this tutorial, but when running outside of Jupyter you do not have this feature automatically. However, the `matplotlib.Figure` class natively has methods to both display and save the visualization. You can call `.show()` on the returned object from `plot_histogram()` to open the image in a new window (assuming your configured matplotlib backend is interactive). Or alternatively you can call `.savefig('out.png')` to save the figure to `out.png`. The `savefig()` method takes a path so you can adjust the location and filename where you're saving the output."
+    "When using the `plot_histogram()` function, it returns a `matplotlib.Figure` for the rendered visualization. Jupyter notebooks understand this return type and render it for us in this tutorial, but when running outside of Jupyter you do not have this feature automatically. However, the `matplotlib.Figure` class natively has methods to both display and save the visualization. You can call `.show()` on the returned object from `plot_histogram()` to open the image in a new window (assuming your configured matplotlib backend is interactive). Or alternatively you can call `.savefig('out.png')` to save the figure to `out.png`. The `savefig()` method takes a path so you can adjust the location and filename where you're saving the output."
    ]
   },
   {