Fix deprecation warning in scala-for-java-programmers.md

[rhysd]

Today I was learning Scala with the text and found scalac reported deprecation warning:

generics.scala:3: warning: procedure syntax is deprecated: instead, add `: Unit =` to explicitly declare `set`'s return type
  def set(value: T) { contents = value }
                    ^
1 warning

This PR fixes the warning.

[julienrf]

Thank you @rhysd!