Fix typo (#26)

examples/nginx.conf

@@ -78,7 +78,7 @@ http {
             default_type text/html;
 
             rewrite_by_lua_block {
-                require("tracer"):startBackendTimer()
+                require("tracer"):start("backend service")
             }
 
             proxy_pass http://127.0.0.1:8080/backend;
@@ -161,4 +161,4 @@ http {
             }
         }
     }
-}
\ No newline at end of file
+}