Fix typo. (#4433)

Author: hotchemi

README.md

@@ -117,7 +117,7 @@ $ cd RxJava/
 $ ./gradlew build
 ```
 
-Futher details on building can be found on the [Getting Started](https://github.com/ReactiveX/RxJava/wiki/Getting-Started) page of the wiki.
+Further details on building can be found on the [Getting Started](https://github.com/ReactiveX/RxJava/wiki/Getting-Started) page of the wiki.
 
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