Update README.txt
Update (broken) reference to new site
@@ -11,4 +11,4 @@ ODF Validator is a tool that validates OpenDocument files and checks them for ce
ODF Validator is available as Java library for command line or embedded use, as well as a web application archive (WAR) file.
Further information can be found under:
-http://incubator.apache.org/odftoolkit/conformance/ODFValidator.html
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+https://odftoolkit.org/conformance/ODFValidator.html
