Fix failing test: having file size more than 42mb

This is small typo fixation. Currently, the test results are:

    $ bundle exec rake test

    =======================================================================================================================
    @1 => ruby -I ./lib -I ./test/lib /mnt/code/github/mongoid-grid_fs/test/mongoid-grid_fs_test.rb
    -----------------------------------------------------------------------------------------------------------------------
    Run options:

    # Running tests:

    [14/17] #<Class:0x007f90b032ec48>#test_00013_iterating-each-chunk-having-file-size-more-than-42mb = 0.02 s
    1) Failure:
    test_00013_iterating-each-chunk-having-file-size-more-than-42mb(#<Class:0x007f90b032ec48>) [/mnt/code/github/mongoid-grid_fs/test/mongoid-grid_fs_test.rb:173]:
    assert()

    Finished tests in 0.568828s, 29.8860 tests/s, 188.1062 assertions/s.
    17 tests, 107 assertions, 1 failures, 0 errors, 0 skips

    ruby -v: ruby 2.0.0p0 (2013-02-24 revision 39474) [x86_64-linux]
    -----------------------------------------------------------------------------------------------------------------------
    @1 <= FAILURE
    -----------------------------------------------------------------------------------------------------------------------

After this patch:

    $ bundle exec rake test

    =======================================================================================================================
    @1 => ruby -I ./lib -I ./test/lib /mnt/code/github/mongoid-grid_fs/test/mongoid-grid_fs_test.rb
    -----------------------------------------------------------------------------------------------------------------------
    Run options:

    # Running tests:

    Finished tests in 1.715598s, 9.9091 tests/s, 63.5347 assertions/s.
    17 tests, 109 assertions, 0 failures, 0 errors, 0 skips

    ruby -v: ruby 2.0.0p0 (2013-02-24 revision 39474) [x86_64-linux]
    -----------------------------------------------------------------------------------------------------------------------
    @1 <= SUCCESS
    -----------------------------------------------------------------------------------------------------------------------