Fix IdentityFunction for LaTeX printing

[sylee957]

References to other Issues or PRs

Brief description of what is fixed or changed

IdentityFunction is inherited from Lambda, but printing is not done properly

Other comments

Release Notes

• printing
  • Fixed IdentityFunction printing for LaTeX.

[sympy-bot]

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• printing
  • Fixed IdentityFunction printing for LaTeX. (#19032 by @sylee957)

This will be added to https://github.com/sympy/sympy/wiki/Release-Notes-for-1.6.

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#### References to other Issues or PRs
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#### Brief description of what is fixed or changed

IdentityFunction is inherited from Lambda, but printing is not done properly

#### Other comments


#### Release Notes

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<!-- BEGIN RELEASE NOTES -->
- printing
  - Fixed IdentityFunction printing for LaTeX.
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Update

The release notes on the wiki have been updated.

[oscarbenjamin]

Should it not just be x rather than x + 1?

[codecov]

Codecov Report

Merging #19032 into master will decrease coverage by 0.026%.
The diff coverage is 100%.

@@             Coverage Diff              @@
##            master   #19032       +/-   ##
============================================
- Coverage   75.777%   75.75%   -0.027%     
============================================
  Files          647      648        +1     
  Lines       168623   168798      +175     
  Branches     39734    39786       +52     
============================================
+ Hits        127778   127866       +88     
- Misses       35280    35366       +86     
- Partials      5565     5566        +1