Update tag_name format for PDF release

Signed-off-by: Bill Traynor <wmat@riscv.org>

Author: wmat

.github/workflows/build-pdf.yml

@@ -64,7 +64,7 @@ jobs:
         uses: softprops/action-gh-release@v2
         with:
           files: ${{ github.workspace }}/build/*.pdf
-          tag_name: ${{ github.event.inputs.version }}
+          tag_name: v${{ github.event.inputs.version }}
           name: Release ${{ github.event.inputs.version }}
           draft: ${{ github.event.inputs.draft }}
           prerelease: ${{ github.event.inputs.prerelease }}