Fixing the Pseudoterminal onDidClose example

[bitekas]

This PR fixes the onDidClose code example.

The current example has a compilcation failure with the error:

[{
	"resource": "extension.ts",
	"owner": "typescript",
	"code": "2322",
	"severity": 8,
	"message": "Type 'Event<TerminalDimensions>' is not assignable to type 'Event<number | void>'.\n  Type 'TerminalDimensions' is not assignable to type 'number | void'.",
	"source": "ts",
	"startLineNumber": 25,
	"startColumn": 4,
	"endLineNumber": 25,
	"endColumn": 14,
	"relatedInformation": [
		{
			"startLineNumber": 11075,
			"startColumn": 3,
			"endLineNumber": 11075,
			"endColumn": 13,
			"message": "The expected type comes from property 'onDidClose' which is declared here on type 'Pseudoterminal'",
			"resource": "node_modules/@types/vscode/index.d.ts"
		}
	]
}]

[Tyriar]

Thanks!