[diff.cpp14.lex] Fix index entry for implementation-defined behavior

Author: jensmaurer

source/compatibility.tex

@@ -843,7 +843,7 @@
 Valid \CppXIV{} code that uses trigraphs may not be valid or may have different
 semantics in this revision of \Cpp{}. Implementations may choose to
 translate trigraphs as specified in \CppXIV{} if they appear outside of a raw
-string literal, as part of the \impldef{mapping from physical source file characters
+string literal, as part of the \impldef{mapping physical source file characters
 to translation character set} mapping from physical source file characters to
 the translation character set.