Fixing the comment-at-end-of-file bug (PR#11982)

collects/scribble/reader.rkt

@@ -50,7 +50,7 @@
 ;; regexps based on the above (more in make-dispatcher)
 (define re:whitespaces   (^px "\\s+"))
 (define re:comment-start (^px ch:comment))
-(define re:comment-line  (^px "[^\n]*\n[ \t]*")) ; like tex's `%'
+(define re:comment-line  (^px "[^\n]*(?:\n|$)[ \t]*")) ; like tex's `%'
 (define re:expr-escape   (^px ch:expr-escape))
 (define re:datums-begin  (^px ch:datums-begin))
 (define re:datums-end    (^px ch:datums-end))

collects/tests/scribble/reader.rkt

@@ -416,6 +416,10 @@ fo@o  -@->  fo@o
 -@->
 (foo "bar " "\n" "baz")
 ---
+hello @; comment at eof
+-@->
+hello
+---
 @foo{bar @;{a balanced comment} baz}
 -@->
 (foo "bar  baz")