Escape + in the expression describing Runtime.Version string

Format of the Runtime.Version string is described as a list of regular expressions [1].

     $VNUM(-$PRE)?\+$BUILD(-$OPT)?
     $VNUM-$PRE(-$OPT)?
     $VNUM(+-$OPT)?

The character + in the last line should be escaped to be consistent with the first line and to avoid confusion with RE quantifiers.

[1] https://docs.oracle.com/en/java/javase/13/docs/api/java.base/java/lang/Runtime.Version.html