Add missing period at sentence end

Author: trueNAHO

src/std_misc/file/read_lines.md

@@ -62,7 +62,7 @@ fn main() {
     }
 }
 
-// The output is wrapped in a Result to allow matching on errors
+// The output is wrapped in a Result to allow matching on errors.
 // Returns an Iterator to the Reader of the lines of the file.
 fn read_lines<P>(filename: P) -> io::Result<io::Lines<io::BufReader<File>>>
 where P: AsRef<Path>, {