fixed broken example and syntax error in module level documentation

//! Errors returned by OpenSSL library.

//!

//! OpenSSL errors are stored in an `ErrorStack`.  Most methods in the crate

/// returns a `Result<T, ErrorStack>` type.

//! returns a `Result<T, ErrorStack>` type.

//!

//! # Examples

//!

//!

//! let an_error = BigNum::from_dec_str("Cannot parse letters");

//! match an_error {

//!     Ok(_)  => _,

//!     Ok(_)  => (),

//!     Err(e) => println!("Parsing Error: {:?}", e),

//! }

//! ```