Fix mongoose docs generation

PUBLISHED_FROM=331821dcd1f7dc8a94581cd8a9b51aa00a89fddc

docs/c-api/mg_util.h/mg_mbuf_append_base64.md

@@ -0,0 +1,10 @@
+---
+title: "mg_mbuf_append_base64()"
+decl_name: "mg_mbuf_append_base64"
+symbol_kind: "func"
+signature: |
+  void mg_mbuf_append_base64(struct mbuf *mbuf, const void *data, size_t len);
+---
+
+Encode `len` bytes starting at `data` as base64 and append them to an mbuf. 
+