[DOCS] Add missing entry in the summary of README.md

[tom-rb]

Missing sequential_values generic test entry in the top summary.

This is a:

• documentation update
• bug fix with no breaking changes
• new functionality
• a breaking change

Description & motivation

Just fixing the README summary. I almost missed the test was already implemented because it was not there.

Checklist

• This code is associated with an Issue which has been triaged and accepted for development.
• I have verified that these changes work locally on the following warehouses (Note: it's okay if you do not have access to all warehouses, this helps us understand what has been covered)
  • BigQuery
  • Postgres
  • Redshift
  • Snowflake
• I followed guidelines to ensure that my changes will work on "non-core" adapters by:
  • dispatching any new macro(s) so non-core adapters can also use them (e.g. the star() source)
  • using the limit_zero() macro in place of the literal string: limit 0
  • using dbt.type_* macros instead of explicit datatypes (e.g. dbt.type_timestamp() instead of TIMESTAMP
• I have updated the README.md (if applicable)
• I have added tests & descriptions to my models (and macros if applicable)
• I have added an entry to CHANGELOG.md

[tom-rb]

Afterthought: this PR can probably be discarded in favor of #486