Update cfx-tool.md

The documentation was not clear on how the autoupdate links are created. The name of the files are also needed in the URLs to specify for the update requests.
Also the cfx xpi -update-link commands need to be on the same line (at least on Windows).
Merge pull request #989 from esgy/patch-1

Fix for bug 888097 Update cfx-tool.md - thanks @esgy!