Update disk-usage.asciidoc (#89709) (#89874)

added missing word

(cherry picked from commit 3e3545551160065e7dfd344cfe3f04da5dbfb345)

Co-authored-by: Brady Vidovic <bradvido@users.noreply.github.com>

docs/reference/how-to/disk-usage.asciidoc

@@ -136,6 +136,6 @@ if fields always occur in the same order.
 [[roll-up-historical-data]]
 === Roll up historical data
 
-Keeping older data can useful for later analysis but is often avoided due to
+Keeping older data can be useful for later analysis but is often avoided due to
 storage costs. You can use data rollups to summarize and store historical data
 at a fraction of the raw data's storage cost. See <<xpack-rollup>>.