Misc typo fixes (#104)

* Add description for COINBASE.
* Fix transfer.

specs/vm/opcodes.md

@@ -54,15 +54,15 @@
   - [CODECOPY: Code copy](#codecopy-code-copy)
   - [CODEROOT: Code Merkle root](#coderoot-code-merkle-root)
   - [CODESIZE: Code size](#codesize-code-size)
-  - [COINBASE](#coinbase)
+  - [COINBASE: Block proposer address](#coinbase-block-proposer-address)
   - [LOADCODE: Load code from an external contract](#loadcode-load-code-from-an-external-contract)
   - [LOG: Log event](#log-log-event)
   - [REVERT: Revert](#revert-revert)
   - [SRW: State read word](#srw-state-read-word)
   - [SRWX: State read 32 bytes](#srwx-state-read-32-bytes)
   - [SWW: State write word](#sww-state-write-word)
   - [SWWX: State write 32 bytes](#swwx-state-write-32-bytes)
-  - [Transfer: Tranfer coins](#transfer-tranfer-coins)
+  - [TRANSFER: Transfer coins](#transfer-transfer-coins)
 - [Cryptographic Opcodes](#cryptographic-opcodes)
   - [ECRECOVER: Signature recovery](#ecrecover-signature-recovery)
   - [KECCAK256: keccak-256](#keccak256-keccak-256)
@@ -915,7 +915,7 @@ Panic if:
 * `$rs + 32 > VM_MAX_RAM`
 * Contract with ID `MEM[$rs, 32]` is not in `tx.inputs`
 
-### COINBASE
+### COINBASE: Block proposer address
 
 |             |                                  |
 | ----------- | -------------------------------- |
@@ -1046,7 +1046,7 @@ Panic if:
 * `$rs + 32 > VM_MAX_RAM`
 * `$fp == 0` (in the script context)
 
-### Transfer: Tranfer coins
+### TRANSFER: Transfer coins
 
 |             |                                                              |
 | ----------- | ------------------------------------------------------------ |