Mention the right flag in the warnings for missing patsyn sigatures

When producing a warning for a missing pattern synonym signature, the -Wmissing-signatures or -Wmissing-exported-signatures flag was being mentioned in the warning. Instead, it should be -Wmissing-pattern-synonym-signatures, as that is the flag that controls the warning.

This inconsistency led to: https://github.com/haskell/haskell-language-server/issues/1205

Fix the logic in warnMissingSignatures to address this.