introduction to numpy: update exercise shape in exercise description

Author: payno

python/numpy/introduction_to_numpy.ipynb

@@ -1953,7 +1953,7 @@
    "source": [
     "## Exercise \n",
     "\n",
-    "Suppose we want to shrink the physical size of a 1000x1000 gray-value image by a factor 4, without changing the type of the pixels.\n",
+    "Suppose we want to shrink the physical size of a 100x100 gray-value image by a factor 4, without changing the type of the pixels.\n",
     "A natural approach is to reduce the number of pixels by a factor 2 in each dimension. This operation is called **binning**. It consists in replacing each 2x2 square in the image by one single pixel value. The next question is: which value should be assigned to each pixel in the new image.\n",
     "\n",
     "\n",