docs: remove spaces from shell variable assignment

The instructions for generating patches are given as shell commands
with variables as placeholders. They use the syntax "SRCTREE= linux",
which is wrong for the Bourne shell family (it runs the command
"linux" with the variable "SRCTREE" set to the empty string).

Remove the spaces to avoid confusion. This breaks the pretty alignment
but helps new contributors who try to run the commands as written.

Signed-off-by: Tom Levy <tomlevy93@gmail.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>