Fixed case issue with 'fault-injection' in documentation

In the 'fault-injection' subdirectory, the first letter F
is capitalized, whereas in index.rst f is lowercase, but in
index.rst all other elements in the same column are capitalized.

Signed-off-by: "Ran.Park" <ranpark@foxmail.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/tencent_3EA07E65C43816C2A8402DC655CF98916B06@qq.com