chore: Spell "unmarshal" consistently with one el

[alexandear]

The PR updates comments and error messages to use unmarshal with one el. Also, configures misspell linter to detects this in the future.

See https://go.dev//wiki/Spelling

[stevehipwell]

LGTM

[stevehipwell]

@gmlewis could you take a look at this?

[gmlewis]

Fantastic. Thank you, @alexandear and @stevehipwell !
LGTM
Merging.

[gmlewis]

Hmmm... I'm not seeing CodeCov run at all.
I'm going to have to investigate once I get back to my computer.

[stevehipwell]

https://github.com/google/go-github/actions/runs/12870419878/job/35881913304?pr=3441#step:8:209

[codecov]

Codecov Report

Attention: Patch coverage is 50.00000% with 1 line in your changes missing coverage. Please review.

Project coverage is 92.26%. Comparing base (2b8c7fa) to head (715c375).
Report is 225 commits behind head on master.

Files with missing lines	Patch %	Lines
github/repos_contents.go	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3441      +/-   ##
==========================================
- Coverage   97.72%   92.26%   -5.46%     
==========================================
  Files         153      174      +21     
  Lines       13390    15023    +1633     
==========================================
+ Hits        13085    13861     +776     
- Misses        215     1068     +853     
- Partials       90       94       +4     

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