fix typo: OkHttpEngine4 -> OkHttp4Engine (#1341)

Author: lauzadis

runtime/protocol/http-client-engines/http-client-engine-okhttp4/README.md

@@ -80,8 +80,8 @@ Caused by: java.lang.ClassNotFoundException: okhttp3.coroutines.ExecuteAsyncKt
 	... 9 more
 ```
 
-It likely means you failed to configure the SDK client to use the `OkHttpEngine4`. 
-Please double-check all of your SDK client configurations to ensure `httpClient = OkHttpEngine4()` is configured,
+It likely means you failed to configure the SDK client to use the `OkHttp4Engine`. 
+Please double-check all of your SDK client configurations to ensure `httpClient = OkHttp4Engine()` is configured,
 and if the problem persists, [open an issue](https://github.com/smithy-lang/smithy-kotlin/issues/new/choose).
 
 ### Android R8 / ProGuard Configuration
@@ -93,4 +93,4 @@ ERROR: R8: Missing class okhttp3.coroutines.ExecuteAsyncKt (referenced from: jav
 You'll need to add the following rule to either `proguard-rules.pro` or `consumer-rules.pro`, depending on your project structure:
 ```
 -dontwarn okhttp3.coroutines.ExecuteAsyncKt
-```
+```