Fix(aot): Properly appends file content (#622)

I haven't been able to get NGC to kick in using different flags as mentioned at d53f25f, this typo might be the cause of the issue.

Author: abdel-ships-it

src/aot/utils.ts

@@ -96,7 +96,7 @@ function getPlatformBrowserFunctionNode(filePath: string, fileContent: string) {
     modifiedFileContent = appendBefore(filePath, modifiedFileContent, callsToPlatformBrowser[0].expression, toAppend);
   } else {
     // just throw it at the bottom
-    modifiedFileContent + toAppend;
+    modifiedFileContent += toAppend;
   }
   return modifiedFileContent;
 }