cleaned up whitespace version of Paul's 'voided' commit:

commit fdec920cb2cf867f4d18030978d0e47104a6ee48
Author: Paul Esch <paul.esch@outlook.com>
Date:   Tue Feb 5 21:16:15 2013 +0100

    restructured section 4.1.10 Voided

    As in my previous merges, I restructured the section for added clarity.
    Please check for completeness and correctness. Note that I changed
    'should' into 'must' in the original sentence that read: 'should mark
    the target statement as voided using the "voided" field.' (line 1226)

    I.m.h.o a core functionality as this is not optional. By all means,
    change back to SHOULD if MUST is too stringent; I did not follow all
    discussion around this.

https://github.com/adlnet/xAPI-Spec/pull/51/files