[typo] fix a typo to make the comment more precise
* The 48 bits available in the struct are divided as follow:
*
* - the 7 most significant bits stand for which segment file the tuple is in
- * (Limit: 128 (2^8)). This also makes up part of the "block number" when
+ * (Limit: 128 (2^7)). This also makes up part of the "block number" when
* interpreted as a heap TID.
*
* - The next 25 bits come from the 25 most significant bits of the row
