Update DialogContext.cs

[JonathanFingold]

• Fix some broken links in the doc XML.
• Opportunistically fix some typos.

This will fix some of the doc build warnings (generated in the bot-docs-pr repo builds).

[fuselabs]

✔️ No Binary Compatibility issues for Microsoft.Bot.Builder.dll compared against version 4.6.3
✔️ No Binary Compatibility issues for Microsoft.Bot.Builder.AI.Luis.dll compared against version 4.6.3
✔️ No Binary Compatibility issues for Microsoft.Bot.Builder.AI.QnA.dll compared against version 4.6.3
✔️ No Binary Compatibility issues for Microsoft.Bot.Builder.ApplicationInsights.dll compared against version 4.6.3
✔️ No Binary Compatibility issues for Microsoft.Bot.Builder.Azure.dll compared against version 4.6.3
✔️ No Binary Compatibility issues for Microsoft.Bot.Builder.Dialogs.dll compared against version 4.6.3
✔️ No Binary Compatibility issues for Microsoft.Bot.Builder.Integration.ApplicationInsights.Core.dll compared against version 4.6.3
✔️ No Binary Compatibility issues for Microsoft.Bot.Builder.Integration.AspNet.Core.dll compared against version 4.6.3
✔️ No Binary Compatibility issues for Microsoft.Bot.Builder.TemplateManager.dll compared against version 4.6.3
✔️ No Binary Compatibility issues for Microsoft.Bot.Builder.Testing.dll compared against version 4.6.3
✔️ No Binary Compatibility issues for Microsoft.Bot.Configuration.dll compared against version 4.6.3
✔️ No Binary Compatibility issues for Microsoft.Bot.Connector.dll compared against version 4.6.3
✔️ No Binary Compatibility issues for Microsoft.Bot.Schema.dll compared against version 4.6.3
✔️ No Binary Compatibility issues for Microsoft.Bot.Streaming.dll compared against version 4.6.3

[coveralls]

Pull Request Test Coverage Report for Build 106742

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 26 unchanged lines in 1 file lost coverage.
• Overall coverage increased (+0.3%) to 78.281%

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Files with Coverage Reduction	New Missed Lines	%
/libraries/Microsoft.Bot.Builder.Dialogs/DialogContext.cs	26	76.89%

Totals
Change from base Build 106737:	0.3%
Covered Lines:	21118
Relevant Lines:	26977

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💛 - Coveralls