doc(rtc_tenclave): simplify reference: std::fs::File → File

(sgx_std ends up with a prefix of `std::untrusted::fs` instead of
`std::fs`)

rtc_tenclave/src/kv_store/fs/std_filer.rs

@@ -1,4 +1,4 @@
-//! [`std::fs::File`] support
+//! [`File`] support
 
 use std::prelude::v1::Vec;