[BUG] RewardWeightUpdate proposal client parse key

[yun-yeo]

Describe the bug

When we create proposal with cmd, we define the proposal.json file format like below

	RewardWeightUpdateProposalJSON struct {
		Title        string    `json:"title" yaml:"title"`
		Description  string    `json:"description" yaml:"description"`
		RewardWeight sdk.Dec   `json:"tax_rate" yaml:"tax_rate"`
		Deposit      sdk.Coins `json:"deposit" yaml:"deposit"`
	}

But, it is mapping RewardWeight to tax_rate json key.

To Reproduce
Try to create RewardWeightUpdateProposal via cmd

Context & versions
v0.4.x

(if applicable) suggested solution
map RewardWeight to reward_weight json key.