vm-subject-tag-prefix inconsistency

[Uday Reddy]

The doc string for `vm-subject-tag-prefix' says that it should not include the square brackets that enclose the subject tags.

However, it doesn't seem to wok unless the regexp includes the square brackets.

This inconsistency should be squared.

[Uday Reddy (reddyuday)]

Fixed in rev. 1467.