Fix typo in link to docker volume driver plugin documentation

[imbuedhope]

There was a missing . in the link which makes it 404.

[CLAassistant]

All committers have signed the CLA.

[codecov-io]

Codecov Report

Merging #1304 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff           @@
##           master   #1304   +/-   ##
======================================
  Coverage    34.2%   34.2%           
======================================
  Files          36      36           
  Lines        2362    2362           
======================================
  Hits          808     808           
  Misses       1451    1451           
  Partials      103     103

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