trivial: Fix typo – alreardy → already

src/validation.cpp

@@ -170,7 +170,7 @@ namespace {
       * BLOCK_FAILED_VALID blocks in a set should be just fine and work just as
       * well.
       *
-      * Because we alreardy walk mapBlockIndex in height-order at startup, we go
+      * Because we already walk mapBlockIndex in height-order at startup, we go
       * ahead and mark descendants of invalid blocks as FAILED_CHILD at that time,
       * instead of putting things in this set.
       */