Fix doc ref

Author: ddeboer

doc/includes/custom-headers.rst

@@ -1,4 +1,4 @@
 This allows you to pass headers that are different between purge requests.
 If you want to add a header to all purge requests, such as ``Authorization``,
-:ref:`HTTP adapter configuration <configure the HTTP adapter>` to use a
+:ref:`configure the HTTP adapter <HTTP adapter configuration>` to use a
 custom HTTP client instead.