fix(create-turbo): Require specific argument for example option (#9147)
### Description
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When we forgot to add the example argument in create-turbo, example was
treated as true.
The error log below makes me a little confused.
``` shell
vscode ➜ /workspaces $ npx create-turbo -e
Need to install the following packages:
create-turbo@2.1.2
Ok to proceed? (y)
? Where would you like to create your Turborepo? ./my-turborepo
? Which package manager do you want to use? npm
>>> Could not locate an example named "true". It could be due to the following:
1. Your spelling of example "true" might be incorrect.
2. You might not be connected to the internet or you are behind a proxy.
```
I couldn't find a reason why the example argument is optional, so I'm
making it require.
```
vscode ➜ /workspaces/turborepo (fix/create-turbo) $ ./packages/create-turbo/dist/cli.js -e
error: option '-e, --example <name>|<github-url>' argument missing
```
### Testing Instructions
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We can check the changes by running the CLI itself.
Alternatively, if testing is required at the Commander level, I can
implement it.
``` shell
./packages/create-turbo/dist/cli.js -e
```