Fix building the Singular docstring dictionaty 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.

Author: antonio-rojas

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