Fixed typo in 13.4

A comment in the example of section 13.4 had 'unnecessary' spelled as 'unnessary'

Author: Ryan-McBride

README.md

@@ -919,7 +919,7 @@
  return name;
  }
 
- // bad - unnessary function call
+ // bad - unnecessary function call
  function(hasName) {
  const name = getName();