Fix typo: rename BDSL to BSDL

[djkazunoko]

I think the file name BDSL might be a typo and should be BSDL.

Reasons

• In https://github.com/ruby/erb/blob/master/LICENSE.txt, it says: see the file BSDL
  • The file BDSL was created in the same PR: Clarify the license of erb #53
• In the Ruby core repository, the file name is BSDL: https://github.com/ruby/ruby/blob/master/BSDL

Because of these points, it seems that BDSL may be an unintended typo and the correct name should be BSDL.

Please let me know if this is intentional or if a rename would be appropriate.

Thank you!

Co-authored-by: @blacktoad30 @kaigokan-op

[djkazunoko]

@k0kubun
One small question unrelated to this change:

The README mentions reporting bugs via Issues, but Issues appear to be disabled for this repository.
I couldn't create an Issue and opened this PR instead.

Is this intentional, or should the README be updated?
https://github.com/ruby/erb/blob/master/README.md?plain=1#L84

[k0kubun]

It is. Updated the README at c0e7d5f.