Fix the broken link of Mnist dataset in beginner.ipynb

This page contains hyperlink for Mnist dataset which is actually asking for credentials now. IMO, for this page the hyperlink may not required.Hence I propose to delete the hyperlink.

Fixes TF ticket #62306

Thanks!

site/en/tutorials/quickstart/beginner.ipynb

@@ -125,7 +125,7 @@
         "\n",
         "## Load a dataset\n",
         "\n",
-        "Load and prepare the [MNIST dataset](http://yann.lecun.com/exdb/mnist/). The pixel values of the images range from 0 through 255. Scale these values to a range of 0 to 1 by dividing the values by `255.0`. This also converts the sample data from integers to floating-point numbers:"
+        "Load and prepare the MNIST dataset. The pixel values of the images range from 0 through 255. Scale these values to a range of 0 to 1 by dividing the values by `255.0`. This also converts the sample data from integers to floating-point numbers:"
       ]
     },
     {