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

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