Fix typos in assert message.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@190034 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/Support/FileSystem.h

@@ -970,7 +970,7 @@ public:
   // modifiers
   /// Goes up one level if Level > 0.
   void pop() {
-    assert(State && "Cannot pop and end itertor!");
+    assert(State && "Cannot pop an end iterator!");
     assert(State->Level > 0 && "Cannot pop an iterator with level < 1");
 
     const directory_iterator end_itr;