Fix comment format in http.h

Functions decl must follow comment to be correctly documented

PUBLISHED_FROM=6ab035cda470c2c23240db4435abd8c241c71b0d

docs/c-api/http.h/mg_fu_fname_fn.md

@@ -0,0 +1,11 @@
+---
+title: "mg_fu_fname_fn"
+decl_name: "mg_fu_fname_fn"
+symbol_kind: "typedef"
+signature: |
+  typedef struct mg_str (*mg_fu_fname_fn)(struct mg_connection *nc,
+                                          struct mg_str fname);
+---
+
+Callback prototype for `mg_file_upload_handler()`. 
+