Correction in package migration for EnvironmentVariables

The new packages are 

net.serenitybdd.model.environment.EnvironmentSpecificConfiguration

net.thucydides.model.environment.SystemEnvironmentVariables

net.thucydides.model.util.EnvironmentVariables

Edited from my mobile. Please verify any typo, but documentation contains this minor typo.

docs/tutorials/migrating_to_serenity_4.md

@@ -93,7 +93,7 @@ If you are using internal Serenity classes, you may need to modify the relevant
 |                               | net.thucydides.core.annotations.*            | net.serenitybdd.annotations.* |
 |                               | Other net.thucydides.core.*                  | net.thucydides.model.*  |
 
-For example, you may need to replace `net.thucydides.core.util.EnvironmentVariables` with `net.thucydides.core.util.EnvironmentVariables`
+For example, you may need to replace `net.thucydides.core.util.EnvironmentVariables` with `net.thucydides.model.util.EnvironmentVariables`
 
 
 ## Step 5: Test Your Changes