Update README.md

Author: malczuuu

README.md

@@ -74,7 +74,7 @@ manager.
        <dependency>
            <groupId>com.github.malczuuu</groupId>
            <artifactId>problem4j-core</artifactId>
-           <version>${problem4j.version}</version>
+           <version>${problem4j-core.version}</version>
        </dependency>
    </dependencies>
    ```
@@ -85,7 +85,7 @@ manager.
    }
    
    dependencies {
-       implementation("com.github.malczuuu:problem4j-core:${problem4j.version}")
+       implementation("com.github.malczuuu:problem4j-core:${problem4j-core.version}")
    }
     ```