block: spelling fixes

Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
Reviewed-by: Eric Blake <eblake@redhat.com>
diff --git a/block.c b/block.c
index 0af890f..cfb7e08 100644
--- a/block.c
+++ b/block.c
@@ -7589,7 +7589,7 @@
     /*
      * Recursion phase: go through all nodes of the graph.
      * Take care of checking that all nodes support changing AioContext
-     * and drain them, builing a linear list of callbacks to run if everything
+     * and drain them, building a linear list of callbacks to run if everything
      * is successful (the transaction itself).
      */
     tran = tran_new();