Mx RFC symbol fix

[samlown]

Describe your changes

Checklist before requesting a review

• I have performed a self-review of my code.
• I have added thorough tests with at least 90% code coverage.
• I've modified or created example GOBL documents the show my changes in use, if appropriate.
• When adding or modifying a tax regime or addon, I've added links to the source of the changes either structured or in the comments.
• I've run go generate . to ensure that Schemas and Regime data are up to date.
• All linter warnings have been reviewed and fixed.
• I've been obsessive with pointer nil checks to avoid panics.
• The CHANGELOG.md has been updated with an overview of my changes.
• Requested a review from @samlown.

[codecov]

Codecov Report

Attention: Patch coverage is 91.25874% with 50 lines in your changes missing coverage. Please review.

Project coverage is 80.12%. Comparing base (834477c) to head (57ed5b5).
Report is 34 commits behind head on addon-de.

Files with missing lines	Patch %	Lines
addons/de/xrechnung/instructions.go	86.15%	6 Missing and 3 partials ⚠️
bill/charges.go	65.21%	7 Missing and 1 partial ⚠️
bill/discounts.go	68.18%	6 Missing and 1 partial ⚠️
org/inbox.go	87.87%	3 Missing and 1 partial ⚠️
addons/de/xrechnung/invoices.go	89.65%	2 Missing and 1 partial ⚠️
addons/eu/en16931/bill.go	93.02%	2 Missing and 1 partial ⚠️
regimes/mx/tax_identity.go	70.00%	2 Missing and 1 partial ⚠️
tax/regime_def.go	82.35%	2 Missing and 1 partial ⚠️
tax/tax.go	82.35%	2 Missing and 1 partial ⚠️
addons/eu/en16931/scenarios.go	0.00%	2 Missing ⚠️
... and 3 more

Additional details and impacted files

@@             Coverage Diff              @@
##           addon-de     #404      +/-   ##
============================================
+ Coverage     78.88%   80.12%   +1.24%     
============================================
  Files           217      229      +12     
  Lines         10722    11175     +453     
============================================
+ Hits           8458     8954     +496     
+ Misses         1909     1858      -51     
- Partials        355      363       +8     

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[samlown]

Replaced with #405