[gitg] Fix the ninja call in one of the building guides

[Alberto Fanjul <albfan src gnome org>]

commit 79f32e9d859ee49d2d6d70956f09c3c85812556d
Author: Grzegorz Szymaszek <gszymaszek short pl>
Date:   Sat May 22 12:23:58 2021 +0200

    Fix the ninja call in one of the building guides

 README.md | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
---
diff --git a/README.md b/README.md
index 02c60712..8961597a 100644
--- a/README.md
+++ b/README.md
@@ -32,7 +32,7 @@ $ git clone https://gitlab.gnome.org/GNOME/gitg.git
 $ cd gitg
 $ meson --prefix=/usr build
 $ ninja -C build
-$ sudo ninja -C install
+$ sudo ninja -C build install
 ```
 
 Alternatively you can build using Flatpak with the org.gnome.gitgDevel.json manifest.