fix typos and grammar in s3 examples

[GautamKumarOffical]

fix typos and grammar in s3 examples

went through the s3 examples directory and fixed a handful of things:

• "s3" -> "S3" where it's used as a noun (cp.rst, mv.rst, rm.rst)
• "all of the bucket owned" -> "all of the buckets owned" (ls.rst)
• "command list objects" -> "command lists objects" (ls.rst)
• "it's original name" -> "its original name" (mv.rst, possessive not contraction)
• "a S3 object" -> "an S3 object" (_concepts.rst, 2 places)
• "EXAMBLE" -> "EXAMPLE" in example presigned URLs (presign.rst)
• "S3 Developer Guide* guide" -> "Amazon S3 Developer Guide*" (presign.rst, redundant word)

relates to #10075