blockchain: align comment and code for getIteratorHead()

[ScottyPoi]

Fixes

 Align comment and code in interface, see https://github.com/ethereumjs/ethereumjs-monorepo/pull/2069/files/48e08c9e6f6ad5956613d067b935aaf07788a23f#r938678329

From meta issue #2105.

Fixes misalignment.

[codecov]

Codecov Report

Merging #2126 (4347f8b) into master (4347f8b) will not change coverage.
The diff coverage is n/a.

❗ Current head 4347f8b differs from pull request most recent head f6935cf. Consider uploading reports for the commit f6935cf to get more accurate results

Flag	Coverage Δ
block	83.81% <0.00%> (ø)
blockchain	80.54% <0.00%> (ø)
client	77.06% <0.00%> (ø)
common	95.31% <0.00%> (ø)
devp2p	82.63% <0.00%> (ø)
ethash	90.81% <0.00%> (ø)
evm	40.72% <0.00%> (ø)
statemanager	84.55% <0.00%> (ø)
trie	81.02% <0.00%> (ø)
util	87.22% <0.00%> (ø)

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[holgerd77]

LGTM, will admin-merge since only code docs touched.