There are two ways to "strikethrough" text in Google Docs.
First open your document.
Then highlight the text you'd like to strikethrough.
You can then go into the format menu.
Hover over "Text"
Then click "Strikethrough"
Even easier, just highlight the text and...
...press [[cmd]] + [[shift]] + X (or [[ctrl]] + [[shift]] + X on Windows)
Pretty nice, no?