developer-guide: Update B (#348)

B was changed in
#317.

Author: sjakobi

docs/developer-guide.md

@@ -103,7 +103,7 @@ Here's a quick overview in order of simplicty:
    it contains *2^B* elements.
 
 The number of bits of the hash value to use at each level of the tree, *B*, is a
-compiled time constant (i.e. 4). In general a larger *B* improves lookup
+compile time constant, currently 5. In general a larger *B* improves lookup
 performance (shallower tree) but hurts modification (large nodes to copy when
 updating the spine of the tree).