embed-rust: Find exported function by name rather than assuming index 0

While the wasm file has only one export, our introduction should set a
good example for how to find functions even for wasm files that have
multiple exports. Find the answer function by name rather than assuming
index 0.

Minor variable name change to avoid having to wrap the line.

docs/embed-rust.md

@@ -62,10 +62,11 @@ let instance = Instance::new(&store, &module, &[]).expect("wasm instance");
 ```
 
 Everything is set. If a WebAssembly module has a start function -- it was run.
-The instance's exports can be used at this point. This wasm file has only one export, so we can index it directly:
+The instance's exports can be used at this point. wasmtime provides functions
+to look up an export by name, and ensure that it's a function:
 
 ```rust
-let answer_fn = instance.exports()[0].func().expect("answer function");
+let answer = instance.find_export_by_name("answer").expect("answer").func().expect("function");
 ```
 
 The exported function can be called using the `call` method. Remember that in most of the cases,
@@ -73,7 +74,7 @@ a `HostRef<_>` object will be returned, so `borrow()` or `borrow_mut()` method h
 specific object. The exported "answer" function accepts no parameters and returns a single `i32` value.
 
 ```rust
-let result = answer_fn.borrow().call(&[]).expect("success");
+let result = answer.borrow().call(&[]).expect("success");
 println!("Answer: {}", result[0].i32());
 ```
 
@@ -94,8 +95,8 @@ fn main() {
     let module = HostRef::new(Module::new(&store, &wasm).expect("wasm module"));
     let instance = Instance::new(&store, &module, &[]).expect("wasm instance");
 
-    let answer_fn = instance.exports()[0].func().expect("answer function");
-    let result = answer_fn.borrow().call(&[]).expect("success");
+    let answer = instance.find_export_by_name("answer").expect("answer").func().expect("function");
+    let result = answer.borrow().call(&[]).expect("success");
     println!("Answer: {}", result[0].i32());
 }
 ```