CI: Use same capital case package name spelling as other dependencies do....

CI: Use same capital case package name spelling as other dependencies do. Seems like pip can get confused about this (maybe just temporarily).

doc-requirements.txt

 sphinx==3.5.3
 sphinx-issues==1.2.0
 sphinx-tabs==3.0.0
-jinja2==3.0.3
+Jinja2==3.0.3
 jupyter