Update declaration of train_X, train_Y, and n_samples in C#

Updated the variable declaration to match the code in the C# jupyter notebook: https://github.com/SciSharp/SciSharpCube/blob/master/home/samples/LinearRegression.ipynb

README.md

@@ -72,6 +72,8 @@ float learning_rate = 0.01f;
 int display_step = 100;
 
 // Sample data
+NDArray train_X, train_Y;
+int n_samples;
 train_X = np.array(3.3f, 4.4f, 5.5f, 6.71f, 6.93f, 4.168f, 9.779f, 6.182f, 7.59f, 2.167f,
              7.042f, 10.791f, 5.313f, 7.997f, 5.654f, 9.27f, 3.1f);
 train_Y = np.array(1.7f, 2.76f, 2.09f, 3.19f, 1.694f, 1.573f, 3.366f, 2.596f, 2.53f, 1.221f,