Errors in Knowledge, Probability, nlp apps notebooks

[nvinayvarma189]

1. knowledge.ipynb - Under the Minimal Consistent Determination section, a line of text is placed in a cell of type 'code' which causes an error.

2. Probability.ipynb - umbrella_prior not defined error. We need to define it with some value.

3. nlp_apps - federallist.txt not found error. The federal list papers need to be added in aima-data.
   Edit: A PR for adding the Federal List papers already exists in aima-data.

[nvinayvarma189]

This can be closed now.