DOC: replace utf-8 quotes by ascii ones

947771b ("MINOR: doc: Add documentation about the FastCGI support")
introduced some UTF-8 in configuration.txt.

Replace the UTF-8 quotes by ASCII.
diff --git a/doc/configuration.txt b/doc/configuration.txt
index e5a2497..8688996 100644
--- a/doc/configuration.txt
+++ b/doc/configuration.txt
@@ -18049,8 +18049,8 @@
     * FCGI_MAX_REQS     The maximum number of concurrent requests this
                         application will accept.
 
-    * FCGI_MPXS_CONNS   “0” if this application does not multiplex connections,
-                        “1” otherwise.
+    * FCGI_MPXS_CONNS   "0" if this application does not multiplex connections,
+                        "1" otherwise.
 
   Some FastCGI applications does not support this feature. Some others close
   the connexion immediately after sending their response. So, by default, this