Restore the (sympy)= target anchor dropped by the seed translation

The source carries `(sympy)=` immediately before the title; the French
seed dropped it — the exact defect class of
QuantEcon/action-translation#65. Nothing references the anchor yet, so
the strict -W build cannot catch it: a dropped anchor only warns once
something links to it, at which point the breakage surfaces in whatever
lecture added the reference, not here.

Found by the new structural-parity guard
(QuantEcon/action-translation#119) during its calibration dry-run over
all 211 source/target lecture pairs — this was the only clean
#65-class hit in the production corpus.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: mmcky

lectures/sympy.md

@@ -41,6 +41,7 @@ translation:
 </div>
 ```
 
+(sympy)=
 # {index}`SymPy <single: SymPy>`
 
 ```{index} single: Python; SymPy