Change body-font-weight-bold to be more bold

[jvalkeal]

Antora's default ui made this change a few weeks ago https://gitlab.com/antora/antora-ui-default/-/commit/41211f75322c45942837413b59173f05f310d3f2.

https://github.com/spring-io/antora-ui-spring/blob/6dd9ccead5a92bc5e129cc4113f2392748846b2f/src/css/vars.css#L46

Essentially value 500 is not bold enough.

[sbrannen]

For reference, the Name and Description headers in the Supported Spring Properties table should be bold.

https://docs.spring.io/spring-framework/reference/appendix.html#appendix-spring-properties