Fix broken nl2br filter example

Fix broken nl2br filter example

This example attempts to handle both Windows-style and Unix-style line endings, but does it wrong; as noted at https://stackoverflow.com/q/12521127/1709587, the regex here will match a single \r\n as if it were a double line-break, which results in all single Windows-style line-breaks getting converted into <p> breaks instead of <br> breaks as intended.

This patch fixes that by applying Alan Moore's proposed fix from https://stackoverflow.com/a/12521544/1709587 of using a negative lookahead so that a \r cannot be counted as a line break if it is followed by a \n (while still allowing the \r\n combination to count as a line break).