Use double backticks for all literal values in trytond documentation
In reStructuredText inline Literal Values should start and end with double back ticks, for example:
This is a ``literal_value``.
See: https://docutils.sourceforge.io/docs/ref/rst/restructuredtext.html#inline-literals
Single backticks indicate the value is Interpreted Text, which is meant to be related, indexed, linked, summarized, or otherwise processed.
See: https://docutils.sourceforge.io/docs/ref/rst/restructuredtext.html#interpreted-text