fix: use github for pod source

Author: triniwiz

packages/canvas/platforms/ios/Podfile

@@ -1,6 +1,7 @@
 use_frameworks!
 platform :ios, '11.0'
-pod 'CanvasNative' , '~> 0.9.19'
+#pod 'CanvasNative' , '~> 0.9.19'
+pod 'CanvasNative', :git => 'https://github.com/NativeScript/canvas.git', :tag => '0.9.21'
 #pod 'CanvasNative', :path => "$(SRCROOT)/../../../../../"