Fixed typo in travis iOS build script

Author: DiligentGraphics

.travis.yml

@@ -82,7 +82,7 @@ script:
   - |
     if [ "$TRAVIS_OS_NAME" = "osx" ]; then 
       if [ "$IOS" = "true" ]; then 
-        cmake .. -DCMAKE_TOOLCHAIN_FILE=../DiligentCore/ios.toolchain.cmake -DIOSPLATFORM=OS -G "Xcode"
+        cmake .. -DCMAKE_TOOLCHAIN_FILE=../DiligentCore/ios.toolchain.cmake -DIOS_PLATFORM=OS -G "Xcode"
       else
         cmake .. -G "Xcode"
       fi