Fix spelling error (retreive for retrieve) in strings

Caught by Lintian analysis of the compiled binaries.

Change-Id: I5d53d596d30ad8fea5d3f8f0116122a8c1aa808c
Reviewed-on: http://gerrit.openafs.org/8865
Reviewed-by: Ken Dreyer <ktdreyer@ktdreyer.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>
Tested-by: Jeffrey Altman <jaltman@your-file-system.com>
Tested-by: BuildBot <buildbot@rampaginggeek.com>