Documentation: object_id_len goes up to 31

The value is stored in a 5-bit field, so we can't support more without
a format version upgrade.

Signed-off-by: Han-Wen Nienhuys <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: hanwen

Documentation/technical/reftable.txt

@@ -443,7 +443,7 @@ Obj block format
 Object blocks are optional. Writers may choose to omit object blocks,
 especially if readers will not use the object name to ref mapping.
 
-Object blocks use unique, abbreviated 2-32 object name keys, mapping to
+Object blocks use unique, abbreviated 2-31 byte object name keys, mapping to
 ref blocks containing references pointing to that object directly, or as
 the peeled value of an annotated tag. Like ref blocks, object blocks use
 the file's standard block size. The abbreviation length is available in