lyxpreview_tools.py: fix a regular expression

lyxpreview_tools.py: fix a regular expression

Three backslashes are needed before a LaTeX command, not one. Before
this commit, the code gave the following error with Python >= 3.6:

  re.error: bad escape \g at position 29

This error was introduced with Python 3.6, as documented [1] by the
following line of documentation:

  Changed in version 3.6: Unknown escapes in pattern consisting of
  '\' and an ASCII letter now are errors.

Although previous Python versions did not give an error, the regular
expression was not working as intended: for example, the "\\n" in
"\\newcommandx" would be interpreted as a new line.

[1] https://docs.python.org/3.6/library/re.html#re.sub