specs: increase number of reserved bytes to two

[rootulp]

Problem

#850 increased the share size and the number of reserved bytes from 1 to 2:

celestia-app/pkg/appconsts/appconsts.go

Lines 28 to 30 in 4535394

	// CompactShareReservedBytes is the number of bytes reserved for the location of
	// the first unit (transaction, ISR, evidence) in a compact share.
	CompactShareReservedBytes = 2

so the specs are now inaccurate

celestia-app/specs/src/specs/consensus.md

Line 58 in 2cc65da

| `SHARE_RESERVED_BYTES` | `uint64` | `1` | `byte` | Bytes reserved at the beginning of each [share](data_structures.md#share). Must be sufficient to represent `SHARE_SIZE`. |

Proposal

1. Update the share size constant from 256 to 512 bytes
2. Update the num reserved bytes constant from 1 to 2
3. Update the diagrams and text in the Share section

[rootulp]

This is blocked on #856

[evan-forbes]

closed by #939