[facet.num.get.virtuals] Add missing punctuation

Fixes ISO/CS comment (C++23 proof)

Author: jensmaurer

source/locales.tex

@@ -2212,20 +2212,20 @@
 where \tcode{loc} is \tcode{str.getloc()}.
 
 \pnum
-The details of this operation occur in three stages
+The details of this operation occur in three stages:
 
 \begin{itemize}
 \item
 Stage 1:
-Determine a conversion specifier
+Determine a conversion specifier.
 \item
 Stage 2:
 Extract characters from \tcode{in} and
 determine a corresponding \tcode{char} value for
 the format expected by the conversion specification determined in stage 1.
 \item
 Stage 3:
-Store results
+Store results.
 \end{itemize}
 
 \pnum