Fixing Typo on Line 106 (#564)

I am part of the Qiskit German translation team and noticed the typo here: "Classification with the an `EstimatorQNN`" This needs to be changed as follows "Classification with an `EstimatorQNN`".

docs/tutorials/02_neural_network_classifier_and_regressor.ipynb

@@ -103,7 +103,7 @@
    "id": "religious-history",
    "metadata": {},
    "source": [
-    "### Classification with the an `EstimatorQNN`\n",
+    "### Classification with an `EstimatorQNN`\n",
     "\n",
     "First we show how an `EstimatorQNN` can be used for classification within a `NeuralNetworkClassifier`. In this context, the `EstimatorQNN` is expected to return one-dimensional output in $[-1, +1]$. This only works for binary classification and we assign the two classes to $\\{-1, +1\\}$."
    ]