all: fix typo

HACKING.md

@@ -256,7 +256,7 @@ attributes to make it work in Markdown renderers that strip "id". -->
 
  *  Use named returns to improve readability of function signatures.
 
- *  When naming a file which defines an enitity, use singular nouns, unless the
+ *  When naming a file which defines an entity, use singular nouns, unless the
     entity is some form of a container for other entities:
 
     ```go