Update hands-on.md to add missing -p read_from_head=true

[ecerulm]

The current Hands On! 101 is missing the -p read_from_head=true= on the "4. Calculate Average Value" example.

If you run the current example you get

[2023/11/24 10:40:10] [ info] [sp] registered task: flb-console:0
[2023/11/24 10:40:10] [ info] [input:tail:tail.0] inotify_fs_add(): inode=5 watch_fd=1 name=/sp-samples-1k.log

and not more output. There is no {"AVG(num)"=>61.230770}. in the output.

By adding the -p read_from_head=true I get the expected output

[2023/11/24 10:42:02] [ info] [sp] registered task: flb-console:0
[0] [1700822522.779461868, {"AVG(num)"=>61.230770}]
[0] [1700822522.780739544, {"AVG(num)"=>47.842106}]
[0] [1700822522.781954929, {"AVG(num)"=>40.647060}]
[0] [1700822522.783120630, {"AVG(num)"=>56.812500}]
[0] [1700822522.783183144, {"AVG(num)"=>99.000000}]
[2023/11/24 10:42:02] [ info] [input:tail:tail.0] inotify_fs_add(): inode=5 watch_fd=1 name=/sp-samples-1k.log

[lecaros]

Hi @ecerulm. Thanks for this contribution.
Could you please review https://github.com/fluent/fluent-bit-docs/blob/master/CONTRIBUTING.md and modify your commit messages accordingly?

[ecerulm]

@lecaros , done! (I think)

[lecaros]

Merged. Thanks for your contribution!