Fix reference tags in README

Author: dphilipson

README.md

@@ -10,10 +10,10 @@ Typesafe string enums in TypeScript.
 * [Usage](#usage)
   - [Creating and using enums](#creating-and-using-enums)
   - [Additional functions](#additional-functions)
-    - [Enum.isType(enum, value)](#enum-istype-enum-value)
-    - [Enum.keys(enum)](#enum-keys-enum)
-    - [Enum.values(enum)](#enum-values-enum)
-    - [Enum.ofKeys(object)](#enum-ofkeys-object)
+    - [Enum.isType(enum, value)](#enumistypeenum-value)
+    - [Enum.keys(enum)](#enumkeysenum)
+    - [Enum.values(enum)](#enumvaluesenum)
+    - [Enum.ofKeys(object)](#enumofkeysobject)
 * [Motivation](#motivation)
   - [Why not built-in enums?](#why-not-built-in-enums)
   - [Why not string literals?](#why-not-string-literals)