Correct doc comment in mergeMaps

[pkqk]

• the comment was wrong
• two panic messages were identical in different paths, making it harder to identify which path was failing

[codecov-commenter]

Codecov Report

Attention: 2 lines in your changes are missing coverage. Please review.

Comparison is base (2db8987) 71.04% compared to head (fd13cd7) 71.07%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #226 +/- ## ========================================== + Coverage 71.04% 71.07% +0.03% ========================================== Files 27 27 Lines 2742 2742 ========================================== + Hits 1948 1949 +1 + Misses 672 671 -1 Partials 122 122 ``` | [Files](https://app.codecov.io/gh/movio/bramble/pull/226?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=movio) | Coverage Δ | | |---|---|---| | [executable\_schema.go](https://app.codecov.io/gh/movio/bramble/pull/226?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=movio#diff-ZXhlY3V0YWJsZV9zY2hlbWEuZ28=) | `77.39% <0.00%> (ø)` | | ... and [1 file with indirect coverage changes](https://app.codecov.io/gh/movio/bramble/pull/226/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=movio)

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