Fix typo in opflow.md

[ovasios]

Merge request type

• [ ] New feature
• [x] Resolves bug
• [x] Documentation
• [ ] Other

Relates to

• [x] OPFLOW
• [ ] SOPFLOW
• [ ] SCOPFLOW
• [ ] TCOPFLOW
• [ ] CMake build system
• [ ] Spack configuration
• [ ] Manual
• [ ] Web docs
• [ ] Other

This MR updates

• [ ] Header files
• [ ] Source code
• [ ] CMake build system
• [ ] Spack configuration
• [ ] Web docs
• [ ] Manual
• [ ] Other

Summary

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Linked Issue(s)

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• Closes #issue
• Resolves #issue

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• Partly related to #issue

• issue

• Partially addresses #issue