docs: complete bumping minimal GNU Make version to 3.81

Commit 37d69ee308 ("docs: bump minimal GNU Make version to 3.81")
changes one entry of GNU make version in the changes.rst, there's still
one more entry saying that one need version 3.80.  Fix that.

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Documentation/process/changes.rst

@@ -70,7 +70,7 @@ computer.
 Make
 ----
 
-You will need GNU make 3.80 or later to build the kernel.
+You will need GNU make 3.81 or later to build the kernel.
 
 Binutils
 --------