Fix

Fix broken markdown code block.

Author: JackPGreen

jackson3/MIGRATING_TO_JACKSON_3.md

@@ -292,7 +292,7 @@ recommend. And all construction of generic `ObjectMapper`:
 
  new ObjectMapper(new YAMLFactory()); // and similar
 
-MUST be converted to format-specific `ObjectMapper`` subtypes:
+MUST be converted to format-specific `ObjectMapper` subtypes:
 
  new YAMLMapper(new YAMLFactory());
  new YAMLMapper(); // same as above