docs: add missing slashes to SChannel client certificate documentation

docs: add missing slashes to SChannel client certificate documentation

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 missed in the transition to markdown
documentation.

Closes #12854