docs: fix typo in option name

Author: dougwilson

README.md

@@ -61,7 +61,7 @@ By default this module will send "index.html" files in response to a request
 on a directory. To disable this set `false` or to supply a new index pass a
 string or an array in preferred order.
 
-##### lastMofified
+##### lastModified
 
 Enable or disable `Last-Modified` header, defaults to true. Uses the file
 system's last modified value.