Fix trac #3818 (bug in modules.xml: double man1 in path), possibly

[nilason]

This might fix #3818. I can't test this fix, but reviewing the code behind https://grass.osgeo.org/addons/grass7/modules.xml and comparing with similar code in g.extension.py:1317 make me believe this can be the cause of the problem.

The modules.xml is generated along addon compilation on server, with log file inluding this double-man1 directory, see eg. https://grass.osgeo.org/addons/grass7/logs/db.join.log:

...
mkdir -p /tmp/.grass7/addons/db.join/docs/html
mkdir -p /tmp/.grass7/addons/db.join/docs/man/man1
mkdir -p /tmp/.grass7/addons/db.join/docs/man/man1/man1
mkdir -p /tmp/.grass7/addons/db.join/scripts
...
VERSION_NUMBER=7.8.3dev /work/src/grass78/dist.x86_64-pc-linux-gnu/tools/g.html2man.py \
"/tmp/.grass7/addons/db.join/docs/html/db.join.html" \ 
"/tmp/.grass7/addons/db.join/docs/man/man1/man1/db.join.1"
...