nghttpx: Make --accesslog-format usage doc help2man friendly

This is unfortunate but help2man behaves badly when there is indented
lines in help messages in commnad-line args.  We removed indentations
to make help2man happy.

src/shrpx.cc

@@ -1129,21 +1129,19 @@ Logging:
                      Specify  format  string   for  access  log.   The
                      default format is combined format.  The following
                      variables are available:
-                       $remote_addr: client IP address
-                       $time_local: local time in Common Log format
-                       $time_iso8601:  local time in ISO 8601 format
-                       $request: HTTP request line
+                     $remote_addr: client IP address.
+                     $time_local: local time in Common Log format.
+                     $time_iso8601: local time in ISO 8601 format.
+                     $request: HTTP request line.
                      $status: HTTP response status code.
-                       $body_bytes_sent: the number of bytes sent
-                                         to client  as response body
+                     $body_bytes_sent: the  number of bytes  sent to
+                     client as response body.
                      $http_<VAR>: value of HTTP request header <VAR>
-                                    where '_' in <VAR> is replaced with
-                                    '-'.
-                       $remote_port: client  port
-                       $server_port: server port
+                     where '_' in <VAR> is replaced with '-'.
+                     $remote_port: client  port.
+                     $server_port: server port.
                      $request_time:   request  processing   time  in
-                                      seconds with milliseconds
-                                      resolution.
+                     seconds with milliseconds resolution.
                      Default: )"
       << DEFAULT_ACCESSLOG_FORMAT << R"(
   --errorlog-file=<PATH>