Fix a typo in doc/regexp.rdoc (remove a backslash)

Remove a backslash `\` in Line 614 of regexp.rdoc.

```ruby
[1] pry(main)> s = 'a' * 25 + 'd' + 'a' * 4 + 'c'
=> "aaaaaaaaaaaaaaaaaaaaaaaaadaaaac"
[2] pry(main)> /(b|a)/ =~ s
=> 0
[3] pry(main)> /(b|a+)/ =~ s
=> 0
[4] pry(main)> /(b|a+)*/ =~ s
=> 0
```