Fix a typo in IntegrationWithExistingApps.md.

Summary:
- [x] Explain the **motivation** for making this change.
- [x] Provide a **test plan** demonstrating that the code is solid.
- [x] (N/A) Match the **code formatting** of the rest of the codebase.
- [x] Target the `master` branch, NOT a "stable" branch.

Fixed a typo in documentation `IntegrationWithExistingApps.md`.

`"Your should see"` -> `"You should see"`

**Screenshots**

Before:

<img width="702" alt="screen shot 2017-05-10 at 12 31 53" src="https://cloud.githubusercontent.com/assets/965994/25917480/13d4bbde-357d-11e7-8977-5e5c2f43e3b9.png">

After:

<img width="699" alt="screen shot 2017-05-10 at 12 33 34" src="https://cloud.githubusercontent.com/assets/965994/25917494/1a0e4c40-357d-11e7-9ba7-8e418e2e3280.png">
Closes #13903

Differential Revision: D5044045

Pulled By: javache

fbshipit-source-id: fa4355cb26cae7de1b7fa12c3afbe9d39d7f230e

Author: yoichitgy

docs/IntegrationWithExistingApps.md

@@ -261,7 +261,7 @@ After you have created your `Podfile`, you are ready to install the React Native
 $ pod install
 ```
 
-Your should see output such as:
+You should see output such as:
 
 ```bash
 Analyzing dependencies