fix(doc/make/msys2): incorrect build instructions

-D CMAKE_BUILD_TYPE=Release does nothing since the default build type is Release. Also, it should be enclosed in quotes "-D CMAKE_BUILD_TYPE=Release", otherwise the shell/cmake fails to properly interpret the arguments.

doc/make/msys2.md

@@ -30,6 +30,6 @@ In the [msys2] shell, execute the following commands.
 git clone https://github.com/leanprover/lean
 cd lean
 mkdir build && cd build
-cmake ../src -G Ninja -D CMAKE_BUILD_TYPE=Release
+cmake ../src -G Ninja
 ninja
 ```