feat(nlp): substitute broken link

subjects/ai/nlp/README.md

@@ -95,7 +95,7 @@ The goal of this exercise is to learn to deal with punctuation. In Natural Langu
 
 # Exercise 3: Tokenization
 
-The goal of this exercise is to learn to tokenize as text. This step is important because it splits the text into token. A token could be a sentence or a word.
+The goal of this exercise is to learn [to tokenize](https://en.wikipedia.org/wiki/Lexical_analysis#Tokenization) as text. This step is important because it splits the text into token. A token could be a sentence or a word.
 
 ```
 text = """Bitcoin is a cryptocurrency invented in 2008 by an unknown person or group of people using the name Satoshi Nakamoto. The currency began use in 2009 when its implementation was released as open-source software."""
@@ -106,8 +106,6 @@ text = """Bitcoin is a cryptocurrency invented in 2008 by an unknown person or g
 
 2. Tokenize this text using `word_tokenize` from NLTK.
 
-_Resources: [How to Get Started with NLP – 6](https://www.analyticsvidhya.com/blog/2019/07how-get-started-nlp-6-unique-ways-perform-tokenization/)_
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