[task.members] Change template parameter name from "R" to "Rcvr".

This makes it the same as in the header synopsis.

Author: tkoeppe

source/exec.tex

@@ -7282,8 +7282,8 @@
 
 \indexlibrarymember{connect}{task}%
 \begin{itemdecl}
-template<@\libconcept{receiver}@ R>
-  @\exposid{state}@<R> connect(R&& recv);
+template<@\libconcept{receiver}@ Rcvr>
+  @\exposid{state}@<Rcvr> connect(Rcvr&& recv);
 \end{itemdecl}
 \begin{itemdescr}
 \pnum
@@ -7292,8 +7292,8 @@
 
 \pnum
 \effects
-Equivalent to: \tcode{return \exposid{state}<R>(exchange(\exposid{handle},
-\{\}), std::forward<R>(recv));}
+Equivalent to: \tcode{return \exposid{state}<Rcvr>(exchange(\exposid{handle},
+\{\}), std::forward<Rcvr>(recv));}
 \end{itemdescr}
 
 \rSec3[task.state]{\tcode{Class template \tcode{task::state}}}