Update "block" to new "text-block"

[rprimus]

Sun Oct 18 17:28:23 BST 2020

Attempting to build the documentation was failing with:

? (defparameter *d* (ccldoc:load-document "ccl:doc;manual;ccl.ccldoc"))
> Error: Unknown form (BLOCK "Tip" (PARA #:|Users of 64-bit platforms may need to pass options such
>                as "-m64" to gcc, may need to give the output library a different
>                extension (such as ".so"), and may need to user slightly different
>                values for other options in order to create an equivalent test
>                library.|))
> While executing: (:INTERNAL CCLDOC::FORM-CLAUSE), in process listener(1).
> Type :GO to continue, :POP to abort, :R for a list of available restarts.
> If continued: Output error text into document and continue
> Type :? for other options.

Investigation revealed block was changed to text-block: https://github.com/Clozure/ccldoc/commit/7c5dcf1513a4e8826718a70df8cfb7415401900e

This PR updates the manual for the above.

[xrme]

Thank you.