libbpf: Correct some typos and syntax issues in usdt doc

Fix some incorrect words, such as "and" -> "an", "it's" -> "its".  Fix
some grammar issues, such as removing redundant "will", "would
complicated" -> "would complicate".

Signed-off-by: Jiawei Zhao <Phoenix500526@163.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20250531095111.57824-1-Phoenix500526@163.com