Fix minor README typo (#5514)

### Description
<!-- Is this a user-visible change? Remember to update RELEASE_NOTES.md
-->
<!-- Is this a bug fix for an issue visible in the latest release?
Mention this in the PR details and ensure a patch release is considered
-->

### How has this been tested?
<!-- Tests can be added to
`Source/IntegrationTests/TestFiles/LitTests/LitTest/` or to
`Source/*.Test/…` and run with `dotnet test` -->

<small>By submitting this pull request, I confirm that my contribution
is made under the terms of the [MIT
license](https://github.com/dafny-lang/dafny/blob/master/LICENSE.txt).</small>

README.md

@@ -21,7 +21,7 @@ This github site contains these materials:
 * the [issue tracker](https://github.com/dafny-lang/dafny/issues)
 * the wiki, including [frequently asked questions](https://github.com/dafny-lang/dafny/wiki/FAQ)
 
-Documentation about the dafny language and tools is located
+Documentation about the Dafny language and tools is located
 [here](https://dafny-lang.github.io/dafny).
 A reference manual is available both [online](https://dafny-lang.github.io/dafny/DafnyRef/DafnyRef) and as [pdf](https://github.com/dafny-lang/dafny/blob/master/docs/DafnyRef/out/DafnyRef.pdf). (A LaTeX version can be produced if needed.)