gh-35678: Fix building the Singular docstring dict when Singular info is built with recent texinfo
    
When building the Singular info with recent texinfo, sections are
numbered using capital letters instead of numbers. This breaks the
current matching logic.
    
URL: #35678
Reported by: Antonio Rojas
Reviewer(s): Matthias Köppe

Author: Release Manager

src/sage/interfaces/singular.py

@@ -2405,7 +2405,7 @@ def generate_docstring_dictionary():
                     a, b = m.groups()
                     node_names[a] = b.strip()
 
-            if line == "6 Index\n":
+            if line in ("6 Index\n", "F Index\n"):
                 in_node = False
 
     nodes[curr_node] = "".join(L)  # last node