Fixed the compile error

extension is final and cannot be changed.

android/src/main/java/fr/greweb/reactnativeviewshot/ViewShot.java

                 final byte[] bytes = os.toByteArray();
                 String data = Base64.encodeToString(bytes, Base64.NO_WRAP);
                 // correct the extension if JPG
+                String imageFormat = extension;
                 if ("jpg".equals(extension)) {
-                    extension = "jpeg";
+                    imageFormat = "jpeg";
                 }
-                data = "data:image/"+extension+";base64," + data;
+                data = "data:image/"+imageFormat+";base64," + data;
                 promise.resolve(data);
             }
         } catch (Exception e) {