fix: Traffic-Sign-Detection/requirements.txt to reduce vulnerabilities

The following vulnerabilities are fixed by pinning transitive dependencies:
- https://snyk.io/vuln/SNYK-PYTHON-MISTUNE-2328096

Author: snyk-bot

Traffic-Sign-Detection/requirements.txt

@@ -50,7 +50,7 @@ llvmlite==0.35.0
 Markdown==3.3.3
 MarkupSafe==1.1.1
 matplotlib==3.3.3
-mistune==0.8.4
+mistune==2.0.1
 nbclient==0.5.1
 nbconvert==6.0.7
 nbformat==5.1.2