doc: binprot spelling fix
from elvio on github
@@ -900,7 +900,7 @@ CAS (16-23): 0x0000000000000000
Extras :
delta (24-31): 0x0000000000000001
initial (32-39): 0x0000000000000000
- exipration (40-43): 0x00000e10
+ expiration (40-43): 0x00000e10
Key : Textual string "counter"
Value : None
</artwork>
