fix issue, closes: facebook#13380

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

I am just fixing typos in the docs.

proofreading

Sign the [CLA][2], if you haven't already.

Small pull requests are much easier to review and more likely to get merged. Make sure the PR does only one thing, otherwise please split it.

Make sure all **tests pass** on both [Travis][3] and [Circle CI][4]. PRs that break tests are unlikely to be merged.

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[1]: https://medium.com/martinkonicek/what-is-a-test-plan-8bfc840ec171#.y9lcuqqi9
[2]: https://code.facebook.com/cla
[3]: https://travis-ci.org/facebook/react-native
[4]
Closes facebook#13382

Differential Revision: D4851786

Pulled By: javache

fbshipit-source-id: 3cb67161653681c061d2e2b4a6a8c14527b8bd9b

docs/IntegrationWithExistingApps.md

@@ -735,7 +735,7 @@ Now your activity is ready to run some JavaScript code.
 
 ### Configure permissions for development error overlay
 
-If your app is targeting the Android `API level 23` or greater, make sure you have the `overlay` permission enabled for the development build. You can check it with `Settings.canDrawOverlays(this);`. This is required in dev builds because react native development errors must be displayed above all the other windows. Due to the new permissions system introduced in the API level 23, the user needs to approve it. This can be acheived by adding the following code to the Activity file in the onCreate() method. OVERLAY_PERMISSION_REQ_CODE is a field of the class which would be responsible for passing the result back to the Activity.
+If your app is targeting the Android `API level 23` or greater, make sure you have the `overlay` permission enabled for the development build. You can check it with `Settings.canDrawOverlays(this);`. This is required in dev builds because react native development errors must be displayed above all the other windows. Due to the new permissions system introduced in the API level 23, the user needs to approve it. This can be achieved by adding the following code to the Activity file in the onCreate() method. OVERLAY_PERMISSION_REQ_CODE is a field of the class which would be responsible for passing the result back to the Activity.
 
 ```java
 if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.M) {

docs/Upgrading.md

@@ -26,7 +26,7 @@ a minimum of conflicts. Under the hood, it consists in 2 phases:
 * Then, the patch is applied on the user's sources.
 
 ### 1. Install Git
-Your project doesn't have to be handled by the Git versioning sytem (could be Mercurial, SVN or none)
+Your project doesn't have to be handled by the Git versioning system (could be Mercurial, SVN or none)
 but Git has to be installed and available in the `PATH`. You can download Git here:
 https://git-scm.com/downloads