[re.submatch.op] Fix inconsistency between declaration and description

of sub_match operator<=>: remove 'constexpr' from declaration, and
change return type in definition from 'bool' to 'auto'.

Author: zygoloid

source/regex.tex

@@ -298,7 +298,7 @@
   template<class BiIter>
     bool operator==(const sub_match<BiIter>& lhs, const sub_match<BiIter>& rhs);
   template<class BiIter>
-    constexpr auto operator<=>(const sub_match<BiIter>& lhs, const sub_match<BiIter>& rhs);
+    auto operator<=>(const sub_match<BiIter>& lhs, const sub_match<BiIter>& rhs);
 
   template<class BiIter, class ST, class SA>
     bool operator==(
@@ -1808,7 +1808,7 @@
 \indexlibrarymember{sub_match}{operator<=>}%
 \begin{itemdecl}
 template<class BiIter>
-  bool operator<=>(const sub_match<BiIter>& lhs, const sub_match<BiIter>& rhs);
+  auto operator<=>(const sub_match<BiIter>& lhs, const sub_match<BiIter>& rhs);
 \end{itemdecl}
 
 \begin{itemdescr}