docs(alias): fix the positional-placeholder example

What does this MR do and why?

The positional-placeholder example in glab alias set --help fails at alias creation, so a user copying it never reaches the run step:

$ glab alias set createissue 'glab create issue --title "$1"'

  ERROR
  Could not create alias: glab create issue --title "$1" does not correspond to a glab command.

Two mistakes in the one line.

The command does not exist. It is glab issue create, not glab create issue. This project uses noun-first grammar.

A non-shell alias value must not include the leading glab. The example directly above it in the same help text already gets this right:

glab alias set mrv 'mr view'
# Run the alias; it expands to "glab mr view -w 123"

Only --shell aliases run through a shell and need the binary named, which is why the igrep example below correctly keeps glab issue list ....

After the change the example works end to end:

$ glab alias set createissue 'issue create --title "$1"'
✓ Added alias.
$ glab createissue "My Issue"
# expands to: glab issue create --title "My Issue"

Test coverage

Verified by running the real binary against an isolated GLAB_CONFIG_DIR, both the failing form and the fixed form, before and after the change. The second example in the help text (glab createissue ...) only reaches the command at all with the fix in place.

go test ./internal/commands/alias/... passes.

docs/source/alias/set.md is regenerated with make gen-docs in the same commit, per AGENTS.md.

Related issues

Closes #8413 (closed)