Fix theme attribute.

Fix the asciidoc attribute pdf-style, which was deprecated.  It should be pdf-theme.

Makefile

@@ -29,7 +29,7 @@ OPTIONS := --trace \
            -a revremark=${REVMARK} \
            -a revdate=${DATE} \
            -a pdf-fontsdir=docs-resources/fonts \
-           -a pdf-style=docs-resources/themes/riscv-pdf.yml \
+           -a pdf-theme=docs-resources/themes/riscv-pdf.yml \
            --failure-level=ERROR
 REQUIRES := --require=asciidoctor-bibtex \
             --require=asciidoctor-diagram \