docs: fix cxx -> cpp
diff --git a/docs/markdown/Design-rationale.md b/docs/markdown/Design-rationale.md
index 67fec0a..4133979 100644
--- a/docs/markdown/Design-rationale.md
+++ b/docs/markdown/Design-rationale.md
@@ -236,7 +236,7 @@
 other build systems. Here's how you would use them.
 
 ```meson
-project('pch demo', 'cxx')
+project('pch demo', 'cpp')
 executable('myapp', 'myapp.cpp', pch : 'pch/myapp.hh')
 ```