Fix casing of 'historical retention'

(cherry picked from commit 1780c89)

ftl/core/deck-config.ftl

@@ -350,7 +350,7 @@ deck-config-ignore-before = Ignore reviews before
 deck-config-optimize-all-tip = You can optimize all presets at once by using the dropdown button next to "Save".
 deck-config-evaluate-button = Evaluate
 deck-config-desired-retention = Desired retention
-deck-config-historical-retention = Historical Retention
+deck-config-historical-retention = Historical retention
 deck-config-smaller-is-better = Smaller numbers indicate a better fit to your review history.
 deck-config-steps-too-large-for-fsrs = When FSRS is enabled, steps of 1 day or more are not recommended.
 deck-config-get-params = Get Params