docs: correct typo in multi-env title (#4386)

website/docs/en/guide/advanced/environments.mdx

@@ -1,4 +1,4 @@
-# MMulti-environment builds
+# Multi-environment builds
 
 Rsbuild supports building outputs for multiple environments at the same time. You can use [environments](/config/environments) to build multiple environments in parallel and set a different Rsbuild config for each environment.