Fix issues in HTML validation

[nanxstats]

From CRAN maintainers:

Please see the problems shown on
https://cran.r-project.org/web/checks/check_results_pkglite.html.

In particular, please see the "Found the following HTML validation problems" NOTEs in the "HTML version of manual" check for the r-devel debian checks results.

R 4.2.0 switched to use HTML5 for documentation pages. Now validation using HTML Tidy finds problems in the HTML generated from your Rd files of the form

• <big> element removed from HTML5
• <center> element removed from HTML5
• <img> attribute "align" not allowed for HTML5
• <img> attribute "hspace" not allowed for HTML5
• <img> attribute "width" has invalid value "120px"
• <img> attribute "width" has invalid value "480px"
• <img> attribute "width" has invalid value "50px"
• <img> attribute "width" has invalid value "72px"

For the first four, please see https://html.spec.whatwg.org/#obsolete-but-conforming-features for info on these: in principle, all can be fixed by using style attributes, e.g.

style='text-align: right;'

instead of align='right' etc., which will work for both the new and old ways of converting Rd to HTML.

For the second four, simply drop the px units: the HTML5 standard asks for a non-negative integer implied to be in CSS pixels.

Note that the problems are found in Rd files auto-generated with roxygen2: to fix it might suffice to re-generate these using the current CRAN version of roxygen2.