Update stringLength.html

Added note in documentaion that you are required to set data-bv-stringlength="true" to use this validator via data attributes. It is included in the changelog, but not in the docs.
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Merge pull request #848 from Relequestual/patch-1

Update stringLength.html to note requirement of data-bv-stringlength="true"
Merge pull request #907 from nghuuphuoc/docs-dev

#848 Update stringLength validator document
#848: Update the stringLength validator doc, thanks to @Relequestual
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