doc fix: Install doc/html/modules/mod/*.html

[michaelortmann]

Found by: michaelortmann
Patch by: michaelortmann
Fixes:

One-line summary:
'doc/html/modules/mod' subdirectory was never created and those html files never copied.

Additional description (if needed):
A while ago we changed doc structure and added the 'doc/html/modules/mod' subdirectory
This PR fixes last fallouts from this change

Test cases demonstrating functionality (if applicable):
make install
Before:

find ~/eggdrop|grep python.html
./doc/html/using/python.html

After:

find ~/eggdrop|grep python.html
./doc/html/using/python.html
./doc/html/modules/mod/python.html