[stringbuf.members, ostringstream.cons, stringstream.cons] Add spaces (#7884)

In 6c12819, this change was done for [stringbuf.cons], but these cases were missed.

Author: jwakely

source/iostreams.tex

@@ -8568,7 +8568,7 @@
 \begin{itemdescr}
 \pnum
 \constraints
-\tcode{is_same_v<SAlloc,Allocator>} is \tcode{false}.
+\tcode{is_same_v<SAlloc, Allocator>} is \tcode{false}.
 
 \pnum
 \effects
@@ -9434,7 +9434,7 @@
 \begin{itemdescr}
 \pnum
 \constraints
-\tcode{is_same_v<SAlloc,Allocator>} is \tcode{false}.
+\tcode{is_same_v<SAlloc, Allocator>} is \tcode{false}.
 
 \pnum
 \effects
@@ -9812,7 +9812,7 @@
 \begin{itemdescr}
 \pnum
 \constraints
-\tcode{is_same_v<SAlloc,Allocator>} is \tcode{false}.
+\tcode{is_same_v<SAlloc, Allocator>} is \tcode{false}.
 
 \pnum
 \effects