Fix README typo for Crystal

[dukenguyenxyz]

Simple Typo fix

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[wing328]

@dukeraphaelng thanks for the PR. You need to update the mustache template as well: https://github.com/OpenAPITools/openapi-generator/blob/master/modules/openapi-generator/src/main/resources/crystal/README.mustache

[wing328]

LGTM