fix bbq memory size estimate (elastic#124022) (elastic#124045)

(cherry picked from commit 76cf99c)

Co-authored-by: weizijun <[email protected]>

Author: benwtrent

docs/reference/how-to/knn-search.asciidoc

@@ -76,7 +76,7 @@ Here are estimates for different element types and quantization levels:
 * `element_type: float`: `num_vectors * num_dimensions * 4`
 * `element_type: float` with `quantization: int8`: `num_vectors * (num_dimensions + 4)`
 * `element_type: float` with `quantization: int4`: `num_vectors * (num_dimensions/2 + 4)`
-* `element_type: float` with `quantization: bbq`: `num_vectors * (num_dimensions/8 + 12)`
+* `element_type: float` with `quantization: bbq`: `num_vectors * (num_dimensions/8 + 14)`
 * `element_type: byte`: `num_vectors * num_dimensions`
 * `element_type: bit`: `num_vectors * (num_dimensions/8)`