fix markdown in F14.md

Summary: Add missing markdown notation

Reviewed By: shixiao

Differential Revision: D7655769

fbshipit-source-id: a0db34846e48afd845b8878de84764d30330b096

folly/container/F14.md

@@ -108,7 +108,7 @@ not found in one of the first 3 chunks.  When searching for a key that is
 not in the map (find miss) the expected probe length at max load factor
 is 1.275 and the P99 probe length is 4.
 
-CHUNK OVERFLOW COUNTS: REFERENCE-COUNTED TOMBSTONES
+## CHUNK OVERFLOW COUNTS: REFERENCE-COUNTED TOMBSTONES
 
 Hash tables with a complex probing strategy (quadratic or double-hashing)
 typically use a tombstone on erase, because it is very difficult to