docs: finally concede that the consensus spelling is "dedupe"

Change documentation and comments to use the word "dedupe," not "dedup"
as found in circa-3.15 kernel sources.

No changes in code or program output--if they used "dedup" before, they
will continue to be spelled "dedup" now.

Signed-off-by: Zygo Blaxell <bees@furryterror.org>

docs/event-counters.md

@@ -140,7 +140,7 @@ The `crawl` event group consists of operations related to scanning btrfs trees t
 dedup
 -----
 
-The `dedup` event group consists of operations that deduplicate data.
+The `dedup` (sic) event group consists of operations that deduplicate data.
 
  * `dedup_bytes`: Total bytes in extent references deduplicated.
  * `dedup_copy`: Total bytes copied to eliminate unique data in extents containing a mix of unique and duplicate data.