Update Doc/library/symtable.rst

Co-authored-by: Irit Katriel <1055913+iritkatriel@users.noreply.github.com>

Author: Yashp002

Doc/library/symtable.rst

@@ -299,7 +299,7 @@ Examining Symbol Tables
 
    .. method:: is_cell()
 
-      Return ``True`` if the symbol is referenced from a nested block.
+      Return ``True`` if the symbol is referenced but not assigned in a nested block.
 
       .. versionadded:: next