docs: fix start of `C` code block
MODIFY
mrbgems/mruby-bigint/README-fgmp.md
mrbgems/mruby-bigint/README-fgmp.md
@@ -10,7 +10,7 @@ with the same API.
For instance, you can link the following trivial program with either
this code, or libgmp.a and get the same results.
-``` C
+```c
#include <stdio.h>
#include "gmp.h"
main()

docs: fix start of `C` code block