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/Intel/SoC/soc.html

@@ -588,7 +588,7 @@ Use the following steps to debug the call to TempRamInit:
 </p>
 <table border="1">
   <tr bgcolor="#c0ffc0">
-    <td>Coreboot Field</td>
+    <td>coreboot Field</td>
     <td>EDK2 Field</td>
     <td>gUefiAcpiBoardInfoGuid</td>
     <td>Use</li>