fix the docker link in the ISSUE_TEMPLATE
Before filing a bug report, note that running node_exporter in Docker is
not recommended, for the reasons detailed in the README:
- https://github.com/prometheus/node_exporter#using-docker
+ https://github.com/prometheus/node_exporter#docker
Finally, also note that node_exporter is focused on *NIX kernels, and the
WMI exporter should be used instead on Windows.
