[std] Replace 'must'.

Partially addresses ISO/CS 017 (C++20 DIS)

Author: jensmaurer

source/lex.tex

@@ -57,8 +57,8 @@
 \pnum
 \indextext{translation!phases|(}%
 The precedence among the syntax rules of translation is specified by the
-following phases.\footnote{Implementations must behave as if these separate phases
-occur, although in practice different phases might be folded together.}
+following phases.\footnote{Implementations behave as if these separate phases
+occur, although in practice different phases can be folded together.}
 
 \begin{enumerate}
 \item