Backport r54763: repair missing spaces after \UNIX.

Doc/api/init.tex

@@ -245,7 +245,7 @@
   program name (set by \cfunction{Py_SetProgramName()} above) and some
   environment variables.  The returned string consists of a series of
   directory names separated by a platform dependent delimiter
-  character.  The delimiter character is \character{:} on \UNIX and Mac OS X,
+  character.  The delimiter character is \character{:} on \UNIX{} and Mac OS X,
   \character{;} on Windows.  The returned string points into
   static storage; the caller should not modify its value.  The value
   is available to Python code as the list

Doc/inst/inst.tex

@@ -296,7 +296,7 @@ being installed is pure Python or contains extensions (``non-pure''):
 \filevar{prefix} and \filevar{exec-prefix} stand for the directories
 that Python is installed to, and where it finds its libraries at
 run-time.  They are always the same under Windows, and very
-often the same under \UNIX and Mac OS X.  You can find out what your Python
+often the same under \UNIX{} and Mac OS X.  You can find out what your Python
 installation uses for \filevar{prefix} and \filevar{exec-prefix} by
 running Python in interactive mode and typing a few simple commands.
 Under \UNIX, just type \code{python} at the shell prompt.  Under

Doc/mac/using.tex

@@ -2,7 +2,7 @@
 \sectionauthor{Bob Savage}{bobsavage@mac.com}
 
 Python on a Macintosh running Mac OS X is in principle very similar to
-Python on any other \UNIX platform, but there are a number of additional
+Python on any other \UNIX{} platform, but there are a number of additional
 features such as the IDE and the Package Manager that are worth pointing out.
 
 Python on Mac OS 9 or earlier can be quite different from Python on