docs: Add missing slashes to CURLOPT_SSLCERT documentation

[JDepooter]

When setting the CURLOPT_SSLCERT option to a certificate thumprint, it is required to have a backslash between the "store location", "store name" and "thumbprint" tokens. These slashes were present in the previous documentation, but were removed in the transition to curldown documentation.

This may indicate a bug in the nroff2cd script, so there may be more missing slashes in other files. Notably, the --cert documentation does not have this problem, despite having almost identical content. Perhaps because it uses single slashes instead of double.

In addition, I'll note that since the files are now .md files, the "<store location><store name><thumbprint>" text is showing up as "" when rendered on github, because angle brackets must be escaped in markdown. The text does show up correctly in the HTML documentation. (It will have the slashes if you view after this change is merged). If the angle brackets in the file are replaced with < and >, the file renders correctly on github. I don't know how adding escaping here would affect the other generated doc formats, so I have not added that change to this pull request. I can do so if desired.

[bagder]

Thanks!