Typo fix unncessary -> unnecessary.

Detected while reading recent CHANGELOG entry but then decided to apply
to entire codebase and docs since why not?

Database/Keys.hs

@@ -226,7 +226,7 @@ isInodeKnown i s = or <$> runReaderIO ((:[]) <$$> SQL.isInodeKnown i s)
  - This is run with a lock held, so only one process can be running this at
  - a time.
  -
- - To avoid unncessary work, the index file is statted, and if it's not
+ - To avoid unnecessary work, the index file is statted, and if it's not
  - changed since last time this was run, nothing is done.
  -
  - A tree is generated from the index, and the diff between that tree

Database/Keys/SQL.hs

@@ -140,7 +140,7 @@ isInodeKnown i s = readDb (isJust <$> selectFirst q [])
 		| sentinalInodesChanged s =
 			-- Note that this select is intentionally not
 			-- indexed. Normally, the inodes have not changed,
-			-- and it would be unncessary work to maintain
+			-- and it would be unnecessary work to maintain
 			-- indexes for the unusual case.
 			[ ContentFilesize ==. inodeCacheToFileSize i
 			, ContentMtime >=. tmin