docs: rewrite first sentence in SDK Intro (#10199)

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## Description

Closes: #10183 

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---

### Author Checklist

*All items are required. Please add a note to the item if the item is not applicable and
please add links to any relevant follow up issues.*

I have...

- [ ] included the correct [type prefix](https://github.com/commitizen/conventional-commit-types/blob/v3.0.0/index.json) in the PR title
- [ ] added `!` to the type prefix if API or client breaking change
- [ ] targeted the correct branch (see [PR Targeting](https://github.com/cosmos/cosmos-sdk/blob/master/CONTRIBUTING.md#pr-targeting))
- [ ] provided a link to the relevant issue or specification
- [ ] followed the guidelines for [building modules](https://github.com/cosmos/cosmos-sdk/blob/master/docs/building-modules)
- [ ] included the necessary unit and integration [tests](https://github.com/cosmos/cosmos-sdk/blob/master/CONTRIBUTING.md#testing)
- [ ] added a changelog entry to `CHANGELOG.md`
- [ ] included comments for [documenting Go code](https://blog.golang.org/godoc)
- [ ] updated the relevant documentation or specification
- [ ] reviewed "Files changed" and left comments if necessary
- [ ] confirmed all CI checks have passed

### Reviewers Checklist

*All items are required. Please add a note if the item is not applicable and please add
your handle next to the items reviewed if you only reviewed selected items.*

I have...

- [ ] confirmed the correct [type prefix](https://github.com/commitizen/conventional-commit-types/blob/v3.0.0/index.json) in the PR title
- [ ] confirmed `!` in the type prefix if API or client breaking change
- [ ] confirmed all author checklist items have been addressed 
- [ ] reviewed state machine logic
- [ ] reviewed API design and naming
- [ ] reviewed documentation is accurate
- [ ] reviewed tests and test coverage
- [ ] manually tested (if applicable)

docs/intro/README.md

@@ -6,7 +6,7 @@ parent:
 
 # Introduction
 
-This folder contains introduction material on the Cosmos SDK.
+This introduction gives a quick start on the Cosmos SDK.
 
 1. [Overview](./overview.md)
 2. [Application-Specific Blockchains](./why-app-specific.md)

docs/migrations/README.md

@@ -6,7 +6,7 @@ parent:
 
 # Migrations
 
-This folder contains all the migration guides to update your app and modules to Cosmos SDK v0.44.
+This document contains all the migration guides to update your app and modules to Cosmos SDK v0.44.
 
 1. [Chain Upgrade Guide to v0.44](./chain-upgrade-guide-044.md)
 2. [REST Endpoints Migration](./rest.md)

docs/run-node/README.md

@@ -6,7 +6,7 @@ parent:
 
 # Running a Node, API and CLI
 
-This folder contains documentation on how to run a node and interact with it.
+This script contains documentation on how to run a node and interact with it.
 
 1. [Setting up the keyring](./keyring.md)
 2. [Running a Node](./run-node.md)