Fix doctest failure

This fixes the failure here: https://travis-ci.org/github/mozilla/mp4parse-rust/jobs/748610958#L725

Author: ChunMinChang

mp4parse_capi/src/lib.rs

@@ -17,7 +17,7 @@
 //!    }
 //! }
 //!
-//! let mut file = std::fs::File::open("../mp4parse/tests/minimal.mp4").unwrap();
+//! let mut file = std::fs::File::open("mp4parse/tests/minimal.mp4").unwrap();
 //! let io = mp4parse_capi::Mp4parseIo {
 //!     read: Some(buf_read),
 //!     userdata: &mut file as *mut _ as *mut std::os::raw::c_void