[ide-1.5.x-gui-refactor] Typo in arduino-core ant build file

Refers to Commit b2b57639ddd0e2a8632d1386ecddd53be20383b5 

In file `arduino-core/build.xml`

```
    <target name="clean" description="Clean out the build directories">
        <delete dir="bin" />
        <delete file="arudino-core.jar" />
    </target>
```

there is a typo in target `clean` 
`arudino-core.jar` should be `arduino-core.jar` instead
