[shotwell] Update INSTALL for ninja

[Jens Georg <jensgeorg src gnome org>]

commit da54c796258d95ad901f6394623ea81e916624b1
Author: Jens Georg <mail jensge org>
Date:   Sat Jun 2 08:36:32 2018 +0200

    Update INSTALL for ninja

 INSTALL | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)
---
diff --git a/INSTALL b/INSTALL
index d9e22d2e..9f69f84d 100644
--- a/INSTALL
+++ b/INSTALL
@@ -12,8 +12,8 @@
       can customize the prefix directory.  Run meson build configure  for
       instructions and other installation options.
 
-    * You may need to execute "make install" as root (i.e. sudo make install)
-      if installing to system directories.
+    * You may need to execute "ninja -C build install" as root
+      (i.e. sudo ninja -C build install) if installing to system directories.
 
     * If you wish to try Shotwell before installing it, you may execute it
       directly from its build directory.  Note that certain desktop integration