doc/specs: Reference the Khronos registry XML
doc/specs: Reference the Khronos registry XML

Years ago Khronos replaced the registry's spec files with newfangled XML
files.  Update the reference in doc/specs/enum.txt accordingly.

Reviewed-by: Emil Velikov <f20878473f414da183da19ee25c8722cc1d711e4@collabora.com>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler