Merge rust-bitcoin/rust-secp256k1#397: Fix example dependency list

3c9dd2fb326b4ffe9680885d1de79904499a4043 Fix example dependency list (Tobin Harding)

Pull request description:

  Example relies on `rand-std` not plain `rand` dependency.

  I do not understand why the following command passes without this patch
  applied

  ```
  cargo test --no-default-features --features=std,rand,bitcoin_hashes
  ```

  But if we put the same code in a standalone binary it fails as expected?

  Since the running of this test is _unusual_ and it is primarily meant as
  an entry point example to the library, remove the mention of "alloc"
  feature and just depend upon "std".

  Fixes: #395

ACKs for top commit:
  apoelstra:
    ACK 3c9dd2fb326b4ffe9680885d1de79904499a4043

Tree-SHA512: 8e7ec7ac846e2916c29b74c7485650e5242ae1141c12c69b50d74efdfee71c11a52cd454231d2a7cdd6f8f683d3ba4369f9bf898a6b9351dc92c2a4e2bd626cd

Author: chain-forgexcr45

src/lib.rs

@@ -42,7 +42,7 @@
 //! trigger any assertion failures in the upstream library.
 //!
 //! ```rust
-//! # #[cfg(all(feature="rand", feature="bitcoin_hashes", any(feature = "alloc", feature = "std")))] {
+//! # #[cfg(all(feature = "std", feature="rand-std", feature="bitcoin_hashes"))] {
 //! use secp256k1::rand::rngs::OsRng;
 //! use secp256k1::{Secp256k1, Message};
 //! use secp256k1::hashes::sha256;