fix broken links in README.md

(cherry picked from commit add89e3)

Author: timfel

README.md

@@ -47,7 +47,7 @@ but this version is what we're testing with in our CI.
 
 ### Polyglot Usage
 
-We have a [document](doc/INTEROP.md) describing how we implement the
+We have a [document](doc/POLYGLOT.md) describing how we implement the
 cross-language interop. This will hopefully give you an idea how to use it.
 
 ### Contributing
@@ -58,7 +58,7 @@ Agreement](http://www.graalvm.org/community/contributors/) for us to able to
 merge your work. Please also take note of our [code of
 conduct](http://www.graalvm.org/community/conduct/) for contributors.
 
-To get you started, we have [written](docs/CONTRIBUTING.md) a bit about the
+To get you started, we have [written a bit](doc/CONTRIBUTING.md) about the
 structure of this interpreter that should show how to fix things or add
 features.