Documentation: Fix spelling of *assumptions*

Change-Id: I36e0e713647cfc0d25e6b4ead81aa212be530afb
Signed-off-by: Paul Menzel <pmenzel@molgen.mpg.de>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/33742
Reviewed-by: Angel Pons <th3fanbus@gmail.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
diff --git a/Documentation/arch/x86/index.md b/Documentation/arch/x86/index.md
index 11d8a4f..d22d31d 100644
--- a/Documentation/arch/x86/index.md
+++ b/Documentation/arch/x86/index.md
@@ -19,7 +19,7 @@
 * x86 payloads are loaded below 4GiB in physical memory and are jumped
   to in *protected mode*
 
-## Assuptions for all stages using the reference implementation
+## Assumptions for all stages using the reference implementation
 * 0-4GiB are identity mapped using 2MiB-pages as WB
 * Memory above 4GiB isn't accessible
 * page tables reside in memory mapped ROM