backtracking -> word_search - replacing the example in doctest (#10188)

* Replacing the generator with numpy vector operations from lu_decomposition.

* Revert "Replacing the generator with numpy vector operations from lu_decomposition."

This reverts commit ad217c6.

* Replacing the example in doctest with a less resource-intensive example.

backtracking/word_search.py

@@ -98,13 +98,7 @@ def word_exists(board: list[list[str]], word: str) -> bool:
     False
     >>> word_exists([["A"]], "A")
     True
-    >>> word_exists([["A","A","A","A","A","A"],
-    ...              ["A","A","A","A","A","A"],
-    ...              ["A","A","A","A","A","A"],
-    ...              ["A","A","A","A","A","A"],
-    ...              ["A","A","A","A","A","B"],
-    ...              ["A","A","A","A","B","A"]],
-    ...             "AAAAAAAAAAAAABB")
+    >>> word_exists([["B", "A", "A"], ["A", "A", "A"], ["A", "B", "A"]], "ABB")
     False
     >>> word_exists([["A"]], 123)
     Traceback (most recent call last):