Fix a trivial comment typo.

Bug: https://code.google.com/p/android/issues/detail?id=170069
Change-Id: Iacd308893561a991971b191d4d10a78ca82753d4
diff --git a/core/main.mk b/core/main.mk
index 13465e3..41e2801 100644
--- a/core/main.mk
+++ b/core/main.mk
@@ -29,7 +29,7 @@
 # TOPDIR is the normal variable you should use, because
 # if we are executing relative to the current directory
 # it can be "", whereas TOP must be "." which causes
-# pattern matching probles when make strips off the
+# pattern matching problems when make strips off the
 # trailing "./" from paths in various places.
 #ifeq ($(TOP),.)
 #TOPDIR :=