Update readme to use ? instead of unwrap (#32)

* Update readme to use ? instead of unwrap

Author: ali-behjati

pyth-sdk-terra/README.md

@@ -14,7 +14,7 @@ For reading the price you just need to call `query_price_feed` function within y
 You can find the contract address and price feed ids in the section [Contracts and Price Feeds](#contracts-and-price-feeds) below.
 
 ```rust
-let price_feed: PriceFeed = query_price_feed(deps.querier, contract_addr, id).unwrap().price_feed;
+let price_feed: PriceFeed = query_price_feed(deps.querier, contract_addr, id)?.price_feed;
 ```
 
 The `PriceFeed` struct contains several useful functions for working with the price.
@@ -27,7 +27,7 @@ For more detailed information, please see the crate documentation.
 Read the current price from a `PriceFeed`: 
 
 ```rust
-let current_price: Price = price_feed.get_current_price().unwrap();
+let current_price: Price = price_feed.get_current_price().ok_or(StdError::not_found("Current Price is not available"))?;
 println!("price: ({} +- {}) x 10^{}", current_price.price, current_price.conf, current_price.expo);
 ```