Fix typo in sqlparser-derive README

[leoyvens]

This is just from reading it, but I think that bit is wrong.

[alamb]

Thank you @leoyvens and @lovasoa

[coveralls]

Pull Request Test Coverage Report for Build 9499265144

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

• For more information on this, see Tracking coverage changes with pull request builds.
• To avoid this issue with future PRs, see these Recommended CI Configurations.
• For a quick fix, rebase this PR at GitHub. Your next report should be accurate.

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 164 unchanged lines in 4 files lost coverage.
• Overall coverage increased (+0.1%) to 89.003%

---

Files with Coverage Reduction	New Missed Lines	%
src/parser/alter.rs	2	66.67%
tests/sqlparser_common.rs	4	89.57%
src/dialect/mod.rs	5	72.08%
src/lib.rs	153	13.01%

Totals
Change from base Build 9439708625:	0.1%
Covered Lines:	26134
Relevant Lines:	29363

---

💛 - Coveralls