[cpp.subst] change "proprocessing file" to "translation unit" (#7293)

The term 'preprocessing translation unit' is defined in [lex.separate]
while the term 'preprocessing file' is never defined, and is
not used anywhere else in the standard.  Prefer to use the
specified term, as it reasonably covers this case.

Author: AlisdairM

source/preprocessor.tex

@@ -1215,7 +1215,7 @@
   macros contained therein have been expanded. The argument's
   preprocessing tokens are completely macro replaced before
   being substituted as if they formed the rest of the preprocessing
-  file with no other preprocessing tokens being available.
+  tranlation unit with no other preprocessing tokens being available.
 \end{itemize}
 \begin{example}
 \begin{codeblock}