Fix documentation on StringFixedBank

[martskins]

Fixed documentation on StringFixedBank, which incorrectly called StringFixed instead of StringFixedBank in the documentation. I also noticed that the documented output for decimal.NewFromFloat(545).StringFixedBank(-1) is 550, but the actual output is 540. I'm not sure if the documentation or the implementation are incorrect in that case, but I modified the documentation to match the current behavior.