INSTALL: update bullet point numbers

INSTALL

@@ -37,14 +37,14 @@ For Unix:
 
 1. mkdir build && cd build
 2. cmake .. -G 'Unix Makefiles'
-2. make -j4
+3. make -j4
 
 For Windows:
 1. Open a command prompt
 2. mkdir build
-2. cd build
-2. cmake ..
-2. cmake --build .
+3. cd build
+4. cmake ..
+5. cmake --build .
 
 For iOS:
 Just check the following project, which deploys a compiler toolchain for different iOS-versions: https://github.com/assimp/assimp/tree/master/port/iOS