Fix wording regarding directories on OS X

@mike-burns pointed out that OS X does not have folders, it has
directories.

README.md

@@ -47,8 +47,8 @@ OS X El Capitan (10.11)
 -----------------------
 
 You may have problems installing Homebrew for the first time on OS X El
-Capitan due to permission changes to the /usr folder (where Homebrew's
-installation folder is located). See the [Homebrew El Capitan troubleshooting instructions](https://github.com/Homebrew/homebrew/blob/master/share/doc/homebrew/El_Capitan_and_Homebrew.md)
+Capitan due to permission changes to the /usr directory (within which the Homebrew
+installation is typically located). See the [Homebrew El Capitan troubleshooting instructions](https://github.com/Homebrew/homebrew/blob/master/share/doc/homebrew/El_Capitan_and_Homebrew.md)
 for steps to resolve the permissions issues that interfere with Homebrew's
 installation.