Fix documentation for default username in res_odbc

Fix documentation for default username in res_odbc

This was previously stated to be "root", but is actually the name of
the context if unspecified.

(closes issue ASTERISK-20258)
Reported by: Stefan x
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Merged revisions 373578 from http://svn.asterisk.org/svn/asterisk/branches/1.8
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Merged revisions 373579 from http://svn.asterisk.org/svn/asterisk/branches/10
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Merged revisions 373580 from http://svn.asterisk.org/svn/asterisk/branches/11

git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@373581 65c4cc65-6c06-0410-ace0-fbb531ad65f3