Tiny fix in documentation code snippet about GtkTreeModelFilter

[cmey]

Age is in 2nd column instead of 1st.

Console output when selecting a row, before fix:
Name: Peter Age: Peter

After this fix:
Name: Peter Age: 20

[coveralls]

Pull Request Test Coverage Report for Build 2690723965

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

• For more information on this, see Tracking coverage changes with pull request builds.
• To avoid this issue with future PRs, see these Recommended CI Configurations.
• For a quick fix, rebase this PR at GitHub. Your next report should be accurate.

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+5.5%) to 70.181%

---

Totals
Change from base Build 2249572744:	5.5%
Covered Lines:	1784
Relevant Lines:	2542

---

💛 - Coveralls

[codecov]

Codecov Report

Merging #649 (c708028) into master (40bacf8) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #649   +/-   ##
=======================================
  Coverage   65.58%   65.58%           
=======================================
  Files          32       32           
  Lines        2720     2720           
=======================================
  Hits         1784     1784           
  Misses        936      936           

---

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