typo

Signed-off-by: Ah-Lun Tang <tang@ahlun.be>
doc: btrees

Signed-off-by: Ah-Lun Tang <tang@ahlun.be>
doc: uitbreiding btrees

Signed-off-by: Ah-Lun Tang <tang@ahlun.be>
doc: extendible hashing

Signed-off-by: Ah-Lun Tang <tang@ahlun.be>