util/gitconfig: Fix commit-msg for BSD grep

BSD grep (on macOS) doesn't like repeated repetition operators, it
throws the error

    grep: repetition-operator operand invalid

This removes the superfluous repetition operator to make the commit-msg
hook work on macOS and other platforms not using GNU grep.

Change-Id: Id0f57d0f14634f7844b889d71342b2982fcadeb2
Signed-off-by: Patrick Elsen <pelsen@xfbs.net>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/38205
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Patrick Georgi <pgeorgi@google.com>

util/gitconfig/commit-msg

@@ -169,7 +169,7 @@ _gen_ChangeId() {
 	git hash-object -t commit --stdin
 }
 
-if ! grep -qi '^[[:space:]]*\+Signed-off-by:' "$MSG"; then
+if ! grep -qi '^[[:space:]]*Signed-off-by:' "$MSG"; then
 	printf "\nError: No Signed-off-by line in the commit message.\n"
 	exit 1
 fi