Fix Tor Browser capitalisation for readability (source interface)

[gonzalo-bulnes]

Status

Ready for review

Description of Changes

When reviewing the usage of user-facing strings borrowed from the Tor Browser (context), I noticed that sometimes the English source would read Tor browser (lowercase). The three occurrences in this PR belong to the source interface.

I propose using Tor Browser instead, primarily to make the instructions more readable to sources.

Considerations

• "Tor browser" has legitimate uses. While "Tor Browser" the name of one such product, you could browse the Tor network with a different browser.
• However, the three sets of instructions make reference specifically to the Tor Browser.
• Using consistently "Tor Browser" as a product name takes advantage of our sensitivity to brands in order to reduce cognitive load of the sources. ("Which browser is this about? Ah! The Tor Browser, I've seen that name!")
• There are other instances where "Tor Browser" is written as a product name, so this would also increase consistency.

Testing

• Strings don't change length so this change should be very straightforward as far as the UI is concerned.
• Check for misspellings.

Deployment

• These strings are internationalized. I didn't modify the .pot file because I assumed strings would be extracted following a dedicated process.

[gonzalo-bulnes]

Note: a staging test in Circle CI timed out. I couldn't re-run the workflow (all options appear disabled to me).

[emkll]

Thanks for the changes @gonzalo-bulnes . The CI timeout is a flake (tracked in #5401). I just restarted the staging job

[zenmonkeykstop]

LGTM based on visual review.