[range.reverse.overview] Add indefinite article

Author: Andreas-Krug

source/ranges.tex

@@ -9922,7 +9922,7 @@
   then \tcode{E.base()}.
 
 \item
-  Otherwise, if \tcode{E} is specialization of \tcode{optional} and \tcode{E}
+  Otherwise, if \tcode{E} is a specialization of \tcode{optional} and \tcode{E}
   models \libconcept{view}, then \tcode{\exposidnc{decay-copy}(E)}.
 
 \item