interfaces: fix udev rules for tun

[bergotorino]

This commit fixes a typo in udev rules that caused following
error/warning message:

invalid key/value pair in file /etc/udev/rules.d/70-snap.bluez.rules on
line 5, starting at character 46 (',')

[zyga]

LGTM, thank you

[zyga]

@mvo5 this looks like a regression in 2.28

[mvo5]

autsch! thanks for finding this.

[codecov-io]

Codecov Report

Merging #4018 into master will decrease coverage by <.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #4018      +/-   ##
==========================================
- Coverage   75.88%   75.87%   -0.01%     
==========================================
  Files         431      431              
  Lines       36913    36913              
==========================================
- Hits        28012    28009       -3     
- Misses       6948     6950       +2     
- Partials     1953     1954       +1

Impacted Files	Coverage Δ
interfaces/builtin/network_control.go	100% <ø> (ø)	⬆️
overlord/snapstate/snapstate.go	80.04% <0%> (-0.25%)	⬇️

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