[saferecl.hp.base] Merge sentences to avoid visual confusion (cplusplus#9313)

Author: timsong-cpp

source/threads.tex

@@ -13723,7 +13723,7 @@
 \pnum
 \expects
 \tcode{*this} is
-a base class subobject of an object \tcode{x} of type \tcode{T}.
+a base class subobject of an object \tcode{x} of type \tcode{T} and
 \tcode{x} is not retired.
 Move-assigning \tcode{d} to \tcode{deleter} does not exit via an exception.