[gardening] Fix recently introduced typo: "referenece" → "reference"

Author: practicalswift

unittests/runtime/Refcounting.cpp

@@ -153,9 +153,9 @@ TEST(RefcountingTest, unowned_retain_release_n) {
   EXPECT_EQ(1u, value);
 }
 
-//////////////////////////////////////////
-// Non-atomic referenece counting tests //
-//////////////////////////////////////////
+/////////////////////////////////////////
+// Non-atomic reference counting tests //
+/////////////////////////////////////////
 
 TEST(RefcountingTest, nonatomic_release) {
   size_t value = 0;