README: fix escape sequence which is non-POSIX

The "\s" escape sequence is non-POSIX and GNU awk gives a warning:

	gawk: cmd. line:69: warning: escape sequence `\s' treated as plain `s'

BSD awk does not give this warning and supports it.
Use the POSIX [[:space:]] character class instead.

References:
- https://pubs.opengroup.org/onlinepubs/9699919799/utilities/awk.html
  The table in the section "Regular Expressions".
- https://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap05.html#tag_05

1 files changed, 1 insertions, 1 deletions

diff --git a/README b/README
index 2877912..31bf46e 100644
--- a/README
+++ b/README
@@ -309,7 +309,7 @@ advertisements, strip tracking parameters and more.
 				$8 = filterlink($8); # enclosure
 
 				# try to remove tracking pixels: <img/> tags with 1px width or height.
-				gsub("<img[^>]*(width|height)[\s]*=[\s]*[\"'"'"' ]?1[\"'"'"' ]?[^0-9>]+[^>]*>", "", $4);
+				gsub("<img[^>]*(width|height)[[:space:]]*=[[:space:]]*[\"'"'"' ]?1[\"'"'"' ]?[^0-9>]+[^>]*>", "", $4);
 
 				print $0;
 			}'