docs/reference/filesystem: Fix typo in block device code example.

docs/reference/filesystem.rst

@@ -124,7 +124,7 @@ interface (i.e. both signatures and behaviours of the
             self.block_size = block_size
             self.data = bytearray(block_size * num_blocks)
 
-        def readblocks(self, block, buf, offset=0):
+        def readblocks(self, block_num, buf, offset=0):
             addr = block_num * self.block_size + offset
             for i in range(len(buf)):
                 buf[i] = self.data[addr + i]