README.md: Fix typos

s/ninji/ninja

Change-Id: I2c92dedb4eeb90761e977d06e5a1789facb12069
Signed-off-by: George Liu <liuxiwei@ieisystem.com>
diff --git a/README.md b/README.md
index 07ed9b1..e9048f3 100644
--- a/README.md
+++ b/README.md
@@ -16,7 +16,7 @@
 
 ```sh
 meson setup build
-ninji -C build
+ninja -C build
 ```
 
 ## Usage