apt.dat parser: indentation fixes; add a comment

- The first place where indentation is fixed was really misleading, at
  least.

- The added comment "'line' may end with an \r character [...]" just
  clarifies something that has been there for ages, to avoid people
  falling into the trap.

src/Airports/apt_loader.cxx

@@ -129,6 +129,7 @@ public:
     throwExceptionIfStreamError(in, "apt.dat", apt_dat);
 
     while ( std::getline(in, line) ) {
+      // 'line' may end with an \r character, see above
       line_num++;
 
       if ( isBlankOrCommentLine(line) )