[container.requirements.general] Remove 'r' from Table 83 and its preamble

We already use 'a' and 'b' as non-const values in swap(a, b) and other
expressions, although https://wg21.link/lwg3028 covers a related issue.

Author: jwakely

source/containers.tex

@@ -85,8 +85,6 @@
 \item
 \tcode{u} denotes an identifier,
 \item
-\tcode{r} denotes value of type (non-const) \tcode{X}, and
-\item
 \tcode{rv} denotes an rvalue of type (non-const) \tcode{X}.
 \end{itemize}
 
@@ -462,7 +460,7 @@
 
 \indexcont{operator=}%
 \begin{itemdecl}
-r = a
+b = a
 \end{itemdecl}
 
 \begin{itemdescr}
@@ -472,7 +470,7 @@
 
 \pnum
 \ensures
-\tcode{r == a}.
+\tcode{b == a}.
 
 \pnum
 \complexity