small misspelling fixes.

src/mark-compact.cc

Index: src/mark-compact.cc
===================================================================
--- src/mark-compact.cc	(revision 14601)
+++ src/mark-compact.cc	(working copy)
@@ -2074,7 +2074,7 @@
     // Finalize the incremental marking and check whether we had an overflow.
     // Both markers use grey color to mark overflowed objects so
     // non-incremental marker can deal with them as if overflow
-    // occured during normal marking.
+    // occurred during normal marking.
     // But incremental marker uses a separate marking deque
     // so we have to explicitly copy its overflow state.
     incremental_marking->Finalize();