[stringbuf.assign] Fix typo ("s" => "rhs"). (#4306)

The parameter of swap is "rhs"; there is no "s" here. (This appears to have been
copy-pasted from [string.swap] where the parameter is named "s".)

Author: StephanTLavavej

source/iostreams.tex

@@ -7810,7 +7810,7 @@
 \expects
 \tcode{allocator_traits<Allocator>::propagate_on_container_swap::value}
 is \tcode{true} or
-\tcode{get_allocator() == s.get_allocator()} is \tcode{true}.
+\tcode{get_allocator() == rhs.get_allocator()} is \tcode{true}.
 
 \pnum
 \effects