Improve wording for exact block number targeted, ref ethereum#649.

EIPS/eip-649.md

@@ -13,7 +13,7 @@
 The average block times are increasing due to the difficulty bomb (also known as the "_ice age_") slowly accelerating. This EIP proposes to delay the difficulty bomb for approximately one and a half year and to reduce the block rewards with the Byzantium fork, the first part of the Metropolis fork.
 
 ## Abstract
-After `BYZANTIUM_FORK_BLKNUM` the client will calculate the difficulty based on a fake block number suggesting the client that the difficulty bomb is adjusting around 3 million blocks later than previously specified with the Homestead fork. Furthermore, block rewards will be adjusted to a base of 3 ETH, uncle and nephew rewards will be adjusted accordingly.
+Starting with `BYZANTIUM_FORK_BLKNUM` the client will calculate the difficulty based on a fake block number suggesting the client that the difficulty bomb is adjusting around 3 million blocks later than previously specified with the Homestead fork. Furthermore, block rewards will be adjusted to a base of 3 ETH, uncle and nephew rewards will be adjusted accordingly.
 
 ## Motivation
 The Casper development and switch to proof-of-stake is delayed, the Ethash proof-of-work should be feasible for miners and allow sealing new blocks every 15 seconds on average for another one and a half years. With the delay of the ice age, there is a desire to not suddenly also increase miner rewards. The difficulty bomb has been known about for a long time and now it's going to stop from happening. In order to maintain stability of the system, a block reward reduction that offsets the ice age delay would leave the system in the same general state as before. Reducing the reward also decreases the likelihood of a miner driven chain split as Ethereum approaches proof-of-stake.