[tab:cpp17.moveassignable] Remove superfluous space. (#4076)

Author: Eelis

source/lib-intro.tex

@@ -1431,7 +1431,7 @@
 \multicolumn{4}{|p{5.3in}|}{
   \tcode{rv}'s state is unspecified.
   \begin{note}
-\ \tcode{rv} must still meet the requirements of the library
+  \tcode{rv} must still meet the requirements of the library
   component that is using it, whether or not \tcode{t} and \tcode{rv} refer to the same object.
   The operations listed in those requirements must
   work as specified whether \tcode{rv} has been moved from or not.