Update docs' dependencies

Author: mdmintz

mkdocs_build/requirements.txt

@@ -18,7 +18,7 @@ importlib-metadata==4.10.0;python_version>="3.6"
 bleach==4.1.0
 jsmin==3.0.0;python_version>="3.6"
 lunr==0.6.1;python_version>="3.6"
-nltk==3.6.6;python_version>="3.6"
+nltk==3.6.7;python_version>="3.6"
 watchdog==2.1.6;python_version>="3.6"
 mkdocs==1.2.3;python_version>="3.6"
 mkdocs-material==8.1.3;python_version>="3.6"