Fixup LWG4172 unique_lock self-move-assignment is broken

Use parens instead of braced-init, as suggested on LWG reflector.

Author: jwakely

source/threads.tex

@@ -7874,7 +7874,7 @@
 \begin{itemdescr}
 \pnum
 \effects
-Equivalent to: \tcode{unique_lock\{std::move(u)\}.swap(*this)}
+Equivalent to: \tcode{unique_lock(std::move(u)).swap(*this)}
 
 \pnum
 \returns
@@ -8365,7 +8365,7 @@
 \begin{itemdescr}
 \pnum
 \effects
-Equivalent to: \tcode{shared_lock\{std::move(sl)\}.swap(*this)}
+Equivalent to: \tcode{shared_lock(std::move(sl)).swap(*this)}
 
 \pnum
 \returns