Fix typo in doc.pl command-line help.
@@ -64,7 +64,7 @@ doc.pl [options]
--cache-only Only use the execution cache - don't attempt to generate it
--pre Pre-build containers for execute elements marked pre
--var Override defined variable
- --var-key Override defined variable and use in cache key
+ --key-var Override defined variable and use in cache key
--doc-path Document path to render (manifest.xml should be located here)
--out Output types (html, pdf, markdown)
--require Require only certain sections of the document (to speed testing)
