Rename image to fix the exception in the Image Document example

Having spaces in the assert name caused the following exception on both iOS and Android:

```
[VERBOSE-2:ui_dart_state.cc(157)] Unhandled Exception: Unable to load asset: PDFs/PSPDFKit Image Example.jpg
#0      PlatformAssetBundle.load (package:flutter/src/services/asset_bundle.dart:221:7)
<asynchronous suspension>
#1      _MyAppState.extractAsset (package:pspdfkit_example/main.dart:53:65)
#2      _MyAppState.showImage (package:pspdfkit_example/main.dart:75:41)
#3      GestureRecognizer.invokeCallback (package:flutter/src/gestures/recognizer.dart:182:24)
#4      TapGestureRecognizer.handleTapUp (package:flutter/src/gestures/tap.dart:486:11)
#5      BaseTapGestureRecognizer._checkUp (package:flutter/src/gestures/tap.dart:264:5)
#6      BaseTapGestureRecognizer.acceptGesture (package:flutter/src/gestures/tap.dart:236:7)
#7      GestureArenaManager.sweep (package:flutter/src/gestures/arena.dart:156:27)
#8      GestureBinding.handleEvent (package:flutter/src/gestures/binding.dart:222:20)
#9      GestureBinding.dispatchEvent (package:flutter/src/gestures/binding.dart:198:22)
#10     GestureBinding<…>
[VERBOSE-2:ui_dart_state.cc(157)] Unhandled Exception: Unable to load asset: PDFs/PSPDFKit Image Example.jpg
#0      PlatformAssetBundle.load (package:flutter/src/services/asset_bundle.dart:221:7)
<asynchronous suspension>
#1      _MyAppState.extractAsset (package:pspdfkit_example/main.dart:53:65)
#2      _MyAppState.showImage (package:pspdfkit_example/main.dart:75:41)
#3      GestureRecognizer.invokeCallback (package:flutter/src/gestures/recognizer.dart:182:24)
#4      TapGestureRecognizer.handleTapUp (package:flutter/src/gestures/tap.dart:486:11)
#5      BaseTapGestureRecognizer._checkUp (package:flutter/src/gestures/tap.dart:264:5)
#6      BaseTapGestureRecognizer.acceptGesture (package:flutter/src/gestures/tap.dart:236:7)
#7      GestureArenaManager.sweep (package:flutter/src/gestures/arena.dart:156:27)
#8      GestureBinding.handleEvent (package:flutter/src/gestures/binding.dart:222:20)
#9      GestureBinding.dispatchEvent (package:flutter/src/gestures/binding.dart:198:22)
#10     GestureBinding<…>
```

Author: Rad Azzouz

example/PDFs/PSPDFKit Image Example.jpg renamed to example/PDFs/PSPDFKit_Image_Example.jpg

@@ -18,7 +18,7 @@ import 'package:pspdfkit_flutter/pspdfkit.dart';
 
 const String _documentPath = 'PDFs/PSPDFKit.pdf';
 const String _lockedDocumentPath = 'PDFs/protected.pdf';
-const String _imagePath = 'PDFs/PSPDFKit Image Example.jpg';
+const String _imagePath = 'PDFs/PSPDFKit_Image_Example.jpg';
 const String _formPath = 'PDFs/Form_example.pdf';
 const String _instantDocumentJsonPath = 'PDFs/Instant/instant-document.json';
 const String _pspdfkitFlutterPluginTitle = 'PSPDFKit Flutter Plugin example app';