Fix option property name

[karlhorky]

The option was first introduced in https://github.com/lukeed/ley/pull/25

Originally the option was proposed to be called --mjs, but before merge the option name was changed to --esm.

[codecov-commenter]

Codecov Report

Base: 48.82% // Head: 48.82% // No change to project coverage :thumbsup:

Coverage data is based on head (30698b0) compared to base (c6b99f7). Patch coverage: 0.00% of modified lines in pull request are covered.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #32 +/- ## ======================================= Coverage 48.82% 48.82% ======================================= Files 2 2 Lines 213 213 ======================================= Hits 104 104 Misses 109 109 ``` | [Impacted Files](https://codecov.io/gh/lukeed/ley/pull/32?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Luke+Edwards) | Coverage Δ | | |---|---|---| | [index.js](https://codecov.io/gh/lukeed/ley/pull/32/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Luke+Edwards#diff-aW5kZXguanM=) | `10.65% <0.00%> (ø)` | | Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Luke+Edwards). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Luke+Edwards)

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[lukeed]

Oops hah