Update query-string-syntax.asciidoc

When i use the syntax with '/' i get an error like :
ElasticSearchParseException[failed to parse date field [2014/05-01], tried both date format [dateOptionalTime], and timestamp number]; nested: IllegalArgumentException[Invalid format: \"2014/05-01\" is malformed at \"/05-01\"]; }]
Then i tried with '-' and it worked.
I you find this modification relevent i could change the documentation for other the api versions
You did an amazing job with elasticSearch.
Have a nice day,
Arthur BABEY
PS : Sorry about my english i'm not fluent.