Typo in path effects dialogue

Migrated from https://bugs.launchpad.net/inkscape/+bug/1780533

The "Mirror Symmetry" path effect has different operation modes. One is called "Free from reflection line", which seems to be a typo, and should be "Free form reflection line".

My exact version is Inkscape 0.92.0 r15299 (on Windows 8.1 x64), but this affects master, so it should be fixed there.