LaTeX: fix the #13525 fix of code-tex markup in docs

[jfbu]

#13525 added some missing backslash to LaTeX mark-up provided in our docs but then the r must be removed. Else:

! LaTeX Error: There's no line here to end.

See the LaTeX manual or LaTeX Companion for explanation.
Type  H <return>  for immediate help.
 ...                                              
                                                  
l.80 \\n
        ewcommand\\sphinxbackoftitlepage{Hello}\\sphinxmaketitle
? X
No pages of output.

[SilverRainZ]

This was my oversight, thank you for correcting it.

[AA-Turner]

@jfbu this is wrong, we still need the 'r'. The backslashes here are escaped for reStructuredText, see:

[jfbu]

@AA-Turner oops, my bad. Very sorry about that. In the past, first time the docs included the extra info on 'maketitle' it must have been ok, then is it the code-tex role which modified the matter? I will check more carefully and make a correct fix.

[jfbu]

@SilverRainZ This was my oversight, sorry about that.