Fix case-sensitive file link in GPIO README.md

[Copilot]

The markdown-link-check CI started failing after switching to Linux runners due to case-sensitivity. The link to RockPi4bPlusDriver.cs used lowercase 'b' but the actual file uses uppercase 'B'.

Changes

• Fixed link path in src/devices/Gpio/README.md: ./Drivers/RockPi4bPlusDriver.cs → ./Drivers/RockPi4BPlusDriver.cs

Original prompt

We recently changed our markdown-lint-check to run on Linux and it looks like that broke some link checks given some casing issues. Can you take a look at this error that we are getting and put up a PR that fixes it?

FILE: ./src/devices/Gpio/README.md
  [✓] Drivers/Sunxi/README.md → Status: 200
  [✓] Drivers/Rockchip/README.md → Status: 200
  [✓] ./Drivers/OrangePi4Driver.cs → Status: 200
  [✓] ./Drivers/OrangePiLiteDriver.cs → Status: 200
  [✓] ./Drivers/OrangePiLite2Driver.cs → Status: 200
  [✓] ./Drivers/OrangePiZeroDriver.cs → Status: 200
  [✓] ./Drivers/OrangePiZero2Driver.cs → Status: 200
  [✖] ./Drivers/RockPi4bPlusDriver.cs → Status: 400 [Error: ENOENT: no such file or directory, access '/home/runner/work/iot/iot/src/devices/Gpio/Drivers/RockPi4bPlusDriver.cs'] {
  errno: -2,
  code: 'ENOENT',
  syscall: 'access',
  path: '/home/runner/work/iot/iot/src/devices/Gpio/Drivers/RockPi4bPlusDriver.cs'
}

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