fix(doc): typo in the header name for Fastly

https://www.fastly.com/documentation/reference/http/http-headers/Surrogate-Key/

Author: 94noni

doc/fastly-configuration.rst

@@ -13,7 +13,7 @@ Tagging
 ~~~~~~~
 
 Fastly supports cache tagging out of the box.
-Configure the tag header to ``Surrogate-Keys``. (``fos_http_cache.tags.response_header`` if you use FOSHttpCacheBundle)
+Configure the tag header to ``Surrogate-Key``. (``fos_http_cache.tags.response_header`` if you use FOSHttpCacheBundle)
 
 Purge
 ~~~~~