[docs] Fix link in UnsupportedOptimizationAttributes.rst. (#31587)

docs/TransparentAttr.rst was recently rewritten as docs/TransparentAttr.md.
Update old link to docs/TransparentAttr.rst.

Fixes Sphinx doc generation.

Author: dan-zheng

docs/proposals/UnsupportedOptimizationAttributes.rst

@@ -16,7 +16,7 @@ requiring manual intervention of the source maintainer.
 -----------------
 
 Force inlining (at least at -O for now). See the discussion in
-:ref:`transparent-attribute`.
+docs/TransparentAttr.md.
 
 @_specialize directs the compiler to specialize generics
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