docs(guide/Interpolation): describe your change...

[greggunner]

docs: Correction to documentation

• What kind of change does this PR introduce? (Bug fix, feature, docs update, ...)
  Typo fix to the function parameters
• What is the current behavior? (You can also link to an open issue here)
  No parameter was declared in the function parameters
• What is the new behavior (if this is a feature change)?
  A parameter is now set so it can be passed and handled correctly in the function
• Does this PR introduce a breaking change?
  No
• Please check if the PR fulfills these requirements
• The commit message follows our guidelines: https://github.com/angular/angular.js/blob/master/CONTRIBUTING.md#commit-message-format
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)
• Other information:

My knowledge is limited here but I believe the function getForm is receiving a variable from the view and should be using that, correct?