Fix FileSystems documentation

[connor-mccarthy]

What this fixes:

This PR resolves several issues I discovered in the documentation on the datasets.filesystems module (this page).

What were the issues?

When I originally tried implementing the code examples I faced several bugs attributed to:

• out of date botocore call signatures
• capitalization errors in the S3FileSystem class name (written as S3Filesystem in one place)
• call signature errors for the S3FileSystem class constructor (uses parameter sessions instead of session in some places) (see s3fs for where this constructor signature is defined)

Testing/reviewing notes

Instructions for generating the documentation locally: here.

[lhoestq]

Good catch ! thanks you for fixing the documentation