`FSharpExpr` documentation references nonexistent module

[tymokvo]

The FSharpExpr type indicates that the active patterns for working with it are in the SourceCodeServices module.

fsharp/src/Compiler/Symbols/Exprs.fsi

Line 61 in a4666e2

/// in 'FSharp.Compiler.SourceCodeServices' can be used to analyze information about the expression.

I believe this module was broken up into other namespaces as part of release 40.0.000.

fsharp/release-notes.md

Lines 122 to 124 in a4666e2

	##### Changes in `FSharp.Compiler.SourceCodeServices`
	* Everything is moved to one of the other namespaces

This should probably be replaced with a reference to FSharp.Compiler.Symbols.FSharpExprPatterns.