Documentation: change coreboot to lowercase

The word 'coreboot' should always be written in lowercase, even at the
start of a sentence.

Unfortunately, some external websites and projects are spelling coreboot
with an uppercase C, so references to those pages can't be changed
without breaking the link.

Change-Id: I79824da8a9ed36a1e4fe23a1711a89535267bf5f
Signed-off-by: Martin Roth <martinroth@google.com>
Reviewed-on: https://review.coreboot.org/20031
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Arthur Heymans <arthur@aheymans.xyz>

Documentation/acpi/gpio.md

@@ -157,7 +157,7 @@ for the GPIO.
 
 
 These are reference implementations and the platforms are free to
-implement these functions in any way they like. Coreboot driver can
+implement these functions in any way they like. coreboot driver can
 then simply call into these functions to generate ACPI AML code to
 get/set/clear any GPIO. In order to decide whether GPIO operations are
 required, driver code can rely either on some config option or read