Simplification of code

In the code example, .ToLookup(p => Convert.ToChar(p.Company.Substring(0, 1)) is an overkill when a simple code such as .ToLookup(p => p.Company[0] achieves same result given that String class supports char indexer to fetch the underlying value.

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* ID: bf6b73de-2f5d-ec35-a010-c54207af6164
* Version Independent ID: b7c784b6-0318-f929-bfaa-e9fcbbd519c4
* Content: [Enumerable.ToLookup Method (System.Linq)](https://docs.microsoft.com/en-us/dotnet/api/system.linq.enumerable.tolookup?view=netframework-4.7.2)
* Content Source: [xml/System.Linq/Enumerable.xml](https://github.com/dotnet/dotnet-api-docs/blob/master/xml/System.Linq/Enumerable.xml)
* Product: **dotnet-api**
* GitHub Login: @dotnet-bot
* Microsoft Alias: **dotnetcontent**