Update speak.py

Addition of a reference about the [Apache's ETag bug](https://issues.apache.org/bugzilla/show_bug.cgi?id=46538) in the corresponding error text according to a [suggestion of Stéphane Bortzmeyer](https://github.com/mnot/redbot/issues/135).
Update speak.py

Replacement of the previous link by another more accurate.
Update speak.py

New wording.
Merge pull request #136 from Sylvhem/master

Addition of a reference about the Apache's ETag bug