fix obscure markup error, wrapping a long line

Doc/inst/inst.tex

@@ -757,8 +757,8 @@ apply, values from ``earlier'' files are overridden by ``later'' files.
 \label{config-filenames}
 
 The names and locations of the configuration files vary slightly across
-platforms.  On \UNIX and Mac OS X, the three configuration files (in the order they
-are processed) are:
+platforms.  On \UNIX{} and Mac OS X, the three configuration files (in
+the order they are processed) are:
 \begin{tableiii}{l|l|c}{textrm}
   {Type of file}{Location and filename}{Notes}
   \lineiii{system}{\filenq{\filevar{prefix}/lib/python\filevar{ver}/distutils/distutils.cfg}}{(1)}