DOC: fix alphabetic ordering for "tune.fail-alloc" setting

Last time I verified, the "f" letter was not between the "l" and the
"m", but between the "e" and the "g", so let's move this entry to the
right place.
diff --git a/doc/configuration.txt b/doc/configuration.txt
index 8a23c0a..26683a5 100644
--- a/doc/configuration.txt
+++ b/doc/configuration.txt
@@ -1554,6 +1554,12 @@
   Each session using compression initializes the compression algorithm with
   this value. The default value is 1.
 
+tune.fail-alloc
+  If compiled with DEBUG_FAIL_ALLOC, gives the percentage of chances an
+  allocation attempt fails. Must be between 0 (no failure) and 100 (no
+  success). This is useful to debug and make sure memory failures are handled
+  gracefully.
+
 tune.h2.header-table-size <number>
   Sets the HTTP/2 dynamic header table size. It defaults to 4096 bytes and
   cannot be larger than 65536 bytes. A larger value may help certain clients
@@ -1662,12 +1668,6 @@
   counts only the pure Lua runtime. If the Lua does a sleep, the sleep is
   not taken in account. The default timeout is 4s.
 
-tune.fail-alloc
-  If compiled with DEBUG_FAIL_ALLOC, gives the percentage of chances an
-  allocation attempt fails. Must be between 0 (no failure) and 100 (no
-  success). This is useful to debug and make sure memory failures are handled
-  gracefully.
-
 tune.maxaccept <number>
   Sets the maximum number of consecutive connections a process may accept in a
   row before switching to other work. In single process mode, higher numbers