Correct some spellings.

Test: mma
Change-Id: Icc49a4ee76f12d302ed18982d334f2f70b7263a8
diff --git a/update_engine_client.cc b/update_engine_client.cc
index bb19632..94253de 100644
--- a/update_engine_client.cc
+++ b/update_engine_client.cc
@@ -396,7 +396,7 @@
     string rollback_partition;
 
     if (!client_->GetRollbackPartition(&rollback_partition)) {
-      LOG(ERROR) << "Error while querying rollback partition availabilty.";
+      LOG(ERROR) << "Error while querying rollback partition availability.";
       return 1;
     }