docs: remove -f option in CONTRIBUTING.md (#180)

Fixes #

In CONTRIBUTING.md, the command for running integration tests does not
work:
```bash
$ bin/run-suite -s ../../../../aws-cdk-cli cli-integ-tests -f 2.179.0-t 'cli-lib deploy'
Error: Cannot use --framework-version with --use-source
    at main (/Users/t-takahashi/Documents/workspace/aws-cdk-cli-testing/packages/@aws-cdk-testing/cli-integ/lib/cli/run-suite.js:85:19)
```
The `-f` option cannot be used with `-s`, so I removed `-f` from the
snippet.


---
By submitting this pull request, I confirm that my contribution is made
under the terms of the Apache-2.0 license

Co-authored-by: Momo Kornher <[email protected]>

Author: tttol

CONTRIBUTING.md

@@ -57,11 +57,10 @@ Run the tests locally by doing the following:
 
 ```shell
 $ cd /path/to/aws-cdk-cli-testing/packages/@aws-cdk-testing/cli-integ
-$ bin/run-suite -s ../../../../aws-cdk-cli cli-integ-tests -f 2.179.0 -t 'TEST NAME'
+$ bin/run-suite -s ../../../../aws-cdk-cli cli-integ-tests -t 'TEST NAME'
 ```
 
-`-s` points to the root of the `aws-cdk-cli` repository, `-f` is the version of
-the framework to use for testing.
+`-s` points to the root of the `aws-cdk-cli` repository.
 
 ### Submitting a PR that involves changes to integration tests