update text on ads panel of brave://rewards

[LaurenWags]

Description

Found while testing #7500

Brave ads should be Brave Ads per #7500 (comment)

Steps to Reproduce

1. Enable Rewards if not already enabled
2. Look at text on Ads panel of brave://rewards

Actual result:

Expected result:

Brave ads should be Brave Ads

Reproduces how often:

easily

Brave version (brave://version info)

Brave	1.5.102 Chromium: 80.0.3987.122 (Official Build) beta (64-bit)
Revision	cf72c4c4f7db75bc3da689cd76513962d31c7b52-refs/branch-heads/3987@{#943}
OS	macOS Version 10.14.6 (Build 18G3020)

Version/Channel Information:

• Can you reproduce this issue with the current release? n/a, change isn't in 1.4.x
• Can you reproduce this issue with the beta channel? yes
• Can you reproduce this issue with the dev channel? yes
• Can you reproduce this issue with the nightly channel? yes

Other Additional Information:

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

Miscellaneous Information:

[kjozwiak]

Verification PASSED on macOS 10.15.3 x64 Catalina using the following build:

Brave | 1.7.64 Chromium: 80.0.3987.132 (Official Build) beta (64-bit)
-- | --
Revision | fcea73228632975e052eb90fcf6cd1752d3b42b4-refs/branch-heads/3987@{#974}
OS | macOS Version 10.15.3 (Build 19D76)

Verification passed on

Brave	1.7.67 Chromium: 80.0.3987.149 (Official Build) beta (64-bit)
Revision	5f4eb224680e5d7dca88504586e9fd951840cac6-refs/branch-heads/3987_137@{#16}
OS	Windows 10 OS Version 1803 (Build 17134.1006)

Verification passed on

Brave	1.7.70 Chromium: 80.0.3987.149 (Official Build) dev (64-bit)
Revision	5f4eb224680e5d7dca88504586e9fd951840cac6-refs/branch-heads/3987_137@{#16}
OS	Ubuntu 18.04 LTS