[format.context] Rephrase recommended practice

Author: jensmaurer

source/utilities.tex

@@ -20622,17 +20622,16 @@
 that appends to \tcode{wstring}.
 
 \pnum
-\begin{note}
+\recommended
 For a given type \tcode{charT},
-implementations are encouraged to provide
+implementations should provide
 a single instantiation of \tcode{basic_format_context}
 for appending to
 \tcode{basic_string<charT>},
 \tcode{vector<charT>},
 or any other container with contiguous storage
 by wrapping those in temporary objects with a uniform interface
 (such as a \tcode{span<charT>}) and polymorphic reallocation.
-\end{note}
 
 \indexlibrarymember{arg}{basic_format_context}%
 \begin{itemdecl}