* doc/org.texi: Decapitalize file name in references to Calc manual.

On a case sensitive file system the references from the HTML Org
manual (e. g. the link href="Calc.html#Embedded-Mode" or
href="../Calc/Embedded-Mode.html#Embedded-Mode") to the HTML Calc
manual (e. g. the file calc.html or calc/Embedded-Mode.html) do not
match.

The info browser did not suffer from this doc bug in org.texi,
it seems to be case-insensitive regarding link target file name.

Michael