Fix wrong cert used in examples/client.rs

[weibin159]

I got this error when trying to run the examples/client.rs

thread 'main' panicked at 'called `Result::unwrap()` on an `Err` value: X509CertVerifyFailed', mbedtls/examples/client.rs:54:6
stack backtrace:
   0: rust_begin_unwind
             at /rustc/d5a82bbd26e1ad8b7401f6a718a9c57c96905483/library/std/src/panicking.rs:575:5
   1: core::panicking::panic_fmt
             at /rustc/d5a82bbd26e1ad8b7401f6a718a9c57c96905483/library/core/src/panicking.rs:64:14
   2: core::result::unwrap_failed
             at /rustc/d5a82bbd26e1ad8b7401f6a718a9c57c96905483/library/core/src/result.rs:1791:5
   3: core::result::Result<T,E>::unwrap
             at /rustc/d5a82bbd26e1ad8b7401f6a718a9c57c96905483/library/core/src/result.rs:1113:23
   4: client::main
             at ./mbedtls/examples/client.rs:49:5
   5: core::ops::function::FnOnce::call_once
             at /rustc/d5a82bbd26e1ad8b7401f6a718a9c57c96905483/library/core/src/ops/function.rs:507:5
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.

I found that it's because of using the wrong cert in examples/client.rs

Fix with:

-   let cert = Arc::new(Certificate::from_pem_multiple(keys::PEM_CERT.as_bytes())?);
+   let cert = Arc::new(Certificate::from_pem_multiple(keys::ROOT_CA_CERT.as_bytes())?);

Same as #123

[weibin159]

The certificate used for unit testing has expired #219

[Taowyoo]

bors r+

[bors]

Build succeeded:

• continuous-integration/travis-ci/push