Consistent JDK versions in CONTRIBUTING.md

Author: elizarov

CONTRIBUTING.md

@@ -81,7 +81,7 @@ to Gradle (in Preferences -> Build, Execution, Deployment -> Build Tools -> Grad
 * JDK 1.6 referred to by the `JDK_16` environment variable. 
   It is OK to have `JDK_16` pointing to a non 1.6 JDK (e.g. `JAVA_HOME`) for external contributions.
 * JDK 1.8 referred to by the `JDK_18` environment variable. Only used by nightly stress-tests. 
-  It is OK to have `JDK_18` to a non 8 JDK (e.g. `JAVA_HOME`) for external contributions.
+  It is OK to have `JDK_18` to a non 1.8 JDK (e.g. `JAVA_HOME`) for external contributions.
 
 For external contributions you can for example add this to your shell startup scripts (e.g. `~/.zshrc`):
 ```shell