LWG3477 Simplify constraints for semiregular-box

Author: burblebee

source/ranges.tex

@@ -3029,7 +3029,7 @@
 { }
 \end{codeblock}
 
-\item If \tcode{\libconcept{assignable_from}<T\&, const T\&>} is not
+\item If \tcode{\libconcept{copyable}<T>} is not
 modeled, the copy assignment operator is equivalent to:
 \begin{codeblock}
 @\exposid{semiregular-box}@& operator=(const @\exposid{semiregular-box}@& that)
@@ -3041,7 +3041,7 @@
 }
 \end{codeblock}
 
-\item If \tcode{\libconcept{assignable_from}<T\&, T>} is not modeled,
+\item If \tcode{\libconcept{movable}<T>} is not modeled,
 the move assignment operator is equivalent to:
 \begin{codeblock}
 @\exposid{semiregular-box}@& operator=(@\exposid{semiregular-box}@&& that)