sagemathgh-39487: Improvement of the rendering of the documentation in polynomial sequence
    
We make a small improvement in the documentation.

### 📝 Checklist

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- [x] The title is concise and informative.
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preview.
    
URL: sagemath#39487
Reported by: MercedesHaiech
Reviewer(s): Travis Scrimshaw

Author: Release Manager

src/sage/rings/polynomial/multi_polynomial_sequence.py

@@ -1290,7 +1290,9 @@ def is_groebner(self, singular=singular):
         forms a Groebner basis if and only if for every element `S` in
         `Syz(LM(I))`:
 
-            `S * G = \sum_{i=0}^{m} h_ig_i ---->_G 0.`
+        .. MATH::
+
+            S \star G = \sum_{i=0}^{m} h_i g_i \longrightarrow_G 0.
 
         EXAMPLES::