git.sh: fix invalid command in git_prep()

"./update project trees" is a leftover from the
old build system design, prior to audits.

this particular call is for when xtree is defined,
which means that a given tree must rely on the given
coreboot tree defined by xtree. the "xtree" tree is
downloaded, so that its crossgcc builds can be re-used
to save time when building targets across many trees.

this is because trees often use identical crossgcc builds.

Signed-off-by: Leah Rowe <leah@libreboot.org>

include/git.sh

@@ -119,7 +119,7 @@ git_prep()
 	[ "$_loc" = "${_loc%/*}" ] || x_ mkdir -p "${_loc%/*}"
 	mv "$tmpgit" "$_loc" || $err "git_prep: !mv $tmpgit $_loc"
 	[ -n "$xtree" ] && [ ! -d "src/coreboot/$xtree" ] && \
-		x_ ./update project trees -f coreboot "$xtree"; return 0
+		x_ ./update trees -f coreboot "$xtree"; return 0
 }
 
 patch_submodules()