update opt-in text below Yes I'm Ready button on brave://rewards

[LaurenWags]

Description

Follow up to #3124

On brave://rewards prior to ever opting into Rewards, the text below the 'Yes I'm Ready' button says "By clicking 'Yes, I'm in!..." when the button above it has different text.

Steps to Reproduce

1. Clean profile with 0.64.95
2. Click on BAT logo. Verify opt in text correctly references the button presented (Join Rewards)
3. Click Learn More link.
4. Verify opt in text below 'Yes I'm in!' buton correctly references that button.
5. Scroll down to the bottom of the page.

Actual result:

Opt in text below the 'Yes I'm ready' button references the wrong button.

Expected result:

"By clicking 'Yes I'm ready', you indicate that you have read and agree to the Terms of Service and Privacy Policy."

Reproduces how often:

easily

Brave version (brave://version info)

Brave	0.65.95 Chromium: 74.0.3729.131 (Official Build) beta(64-bit)
Revision	518a41c1fa7ce1c8bb5e22346e82e42b4d76a96f-refs/branch-heads/3729@{#954}
OS	Mac OS X

Version/Channel Information:

• Can you reproduce this issue with the current release? n/a
• Can you reproduce this issue with the beta channel? yes
• Can you reproduce this issue with the dev channel? unsure
• Can you reproduce this issue with the nightly channel? unsure

Other Additional Information:

• Does the issue resolve itself when disabling Brave Shields? n/a
• Does the issue resolve itself when disabling Brave Rewards? n/a
• Is the issue reproducible on the latest version of Chrome? n/a

Miscellaneous Information:

[btlechowski]

Reproduced on

Brave	0.65.97 Chromium: 74.0.3729.131 (Official Build) beta(64-bit)
Revision	518a41c1fa7ce1c8bb5e22346e82e42b4d76a96f-refs/branch-heads/3729@{#954}
OS	Ubuntu 18.04 LTS

[jdkuki]

Reproduced on

Brave	0.67.42 Chromium: 75.0.3770.38 (Developer Build) (64-bit)
Revision	3860105745f2b12537da9e9f048f14c3f52ba970-refs/branch-heads/3770@{#618}
OS	Mac OS X

[LaurenWags]

Verified passed with

Brave	0.67.106 Chromium: 75.0.3770.100 (Official Build) beta(64-bit)
Revision	cd0b15c8b6a4e70c44e27f35c37a4029bad3e3b0-refs/branch-heads/3770@{#1033}
OS	Mac OS X

• Verified STR from description

Verification passed on

Brave	0.67.108 Chromium: 75.0.3770.100 (Official Build) beta (64-bit)
Revision	cd0b15c8b6a4e70c44e27f35c37a4029bad3e3b0-refs/branch-heads/3770@{#1033}
OS	Windows 10 OS Version 1803 (Build 17134.523)

• Verified the STR from the description

Verification passed on

Brave	0.67.110 Chromium: 75.0.3770.100 (Official Build) beta (64-bit)
Revision	cd0b15c8b6a4e70c44e27f35c37a4029bad3e3b0-refs/branch-heads/3770@{#1033}
OS	Linux

• Verified the STR from the description