Fix code examples in README

1. Change variable name in example from 'dateString' to 'dateStr' to match
   other occurrences
2. Dust helper parameters (like locale or currency identifiers) must be passed
   as strings. Numeric values like 40000.004 should be passed as strings as
   well, otherwise decimals with leading zeros get truncated (e.g. 40000.4).
Yikes, ok.