Fix mongoose docs generation

PUBLISHED_FROM=331821dcd1f7dc8a94581cd8a9b51aa00a89fddc

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

@@ -0,0 +1,14 @@
+---
+title: "mg_open()"
+decl_name: "mg_open"
+symbol_kind: "func"
+signature: |
+  int mg_open(const char *path, int flag, int mode);
+---
+
+Opens the given file and returns a file stream.
+
+`path` should be UTF8 encoded.
+
+Return value is the same as for the `open()` syscall. 
+