Fix some minor issues in escape documentation.

[AndrolGenhald]

• Remove forward slash and backslash from the list of escaped characters in the book since they aren't escaped
• Update the OWASP link, which used to include forward slash in the list of recommended escapes, but no longer does (askama's HTML escaper currently matches the OWASP example)
• Update the escape filter documentation to remove the incomplete list of escaped characters.
• Add example for specifying alternate escaper to escape filter documentation.

[djc]

Thanks!