src.conf.5: Fix "incompatibility" typo

MFC after: 3 days
Reviewed by: ziaee
Signed-off-by: Christos Longros <chris.longros@gmail.com>
Closes: https://github.com/freebsd/freebsd-src/pull/2035

(cherry picked from commit 102fed041d7f9aec6b962e215f53347a7e5d1a20)