fix typo (thanks Ilya Bogdanov!)

nlp_course/text_classification.html

@@ -2330,7 +2330,7 @@ <h3><u><font face="arial" color="#79a123">P(x|y=k)</font>: use the "naive" assum
         appeared in documents of class \(k\) among all tokens in these documents:
         \[P(x_i|y=k)=\frac{N(x_i, y=k)}{\sum\limits_{t=1}^{|V|}N(x_t, y=k)},\]
         where \(N(x_i, y=k)\) is the number of times
-        the token \(x_i\) appeared in a document
+        the token \(x_i\) appeared in documents
         with the label \(k\), \(V\) is the vocabulary (more generally, a set of all possible features).
     </p>
 
@@ -4360,7 +4360,7 @@ <h2>More in the paper</h2>
 
         </div>
 </div>
-</div>
+