fix(book): change recommended --authrpc.port value from 9999 to 8551

[shekhirin]

We recommend to run Lighthouse with execution endpoint on port 8551 later, which is a bit confusing if people just follow the book:

reth/book/run/mainnet.md

Lines 49 to 54 in 3c292b4

	```bash
	RUST_LOG=info lighthouse bn \
	--checkpoint-sync-url https://mainnet.checkpoint.sigp.io \
	--execution-endpoint http://localhost:8551 \
	--execution-jwt /path/to/secret
	```

[mattsse]

lgtm, we should use default port here

[codecov]

Codecov Report

Merging #3599 (3c292b4) into main (a932e2f) will decrease coverage by 0.01%.
The diff coverage is n/a.

see 8 files with indirect coverage changes

Flag	Coverage Δ
integration-tests	16.11% <ø> (+<0.01%)	⬆️
unit-tests	64.00% <ø> (-0.01%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Components	Coverage Δ
reth binary	26.41% <ø> (ø)
blockchain tree	81.25% <ø> (ø)
pipeline	86.91% <ø> (ø)
storage (db)	73.48% <ø> (ø)
trie	95.09% <ø> (ø)
txpool	49.82% <ø> (ø)
networking	77.90% <ø> (-0.02%)	⬇️
rpc	58.01% <ø> (ø)
consensus	62.69% <ø> (ø)
revm	34.95% <ø> (ø)
payload builder	6.83% <ø> (ø)
primitives	88.28% <ø> (-0.02%)	⬇️