[cpp.subst] Fix typo

Author: Andreas-Krug

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
-  tranlation unit with no other preprocessing tokens being available.
+  translation unit with no other preprocessing tokens being available.
 \end{itemize}
 \begin{example}
 \begin{codeblock}