doc: Correct fragment in doc comment
Otherwise gi-docgen will fail the build with:
[7/8] Generating doc/sdk/libide-doc with a custom command
FAILED: doc/sdk/libide
/nix/store/nz211kmdc2by42mm4xccskyw5c1chspd-gi-docgen-2022.1/bin/gi-docgen generate --quiet --fatal-warnings --add-include-path=/home/jtojnar/Projects/gnome-builder/build/doc/sdk/../../src --config=doc/sdk/libide.toml --output-dir=doc/sdk/libide --no-namespace-dir --content-dir=/home/jtojnar/Projects/gnome-builder/doc/sdk src/Ide-42.0.gir
WARNING: Invalid fragment for 'Gio.SubprocessFlags': it should be flags
[enum@Gio.SubprocessFlags] to use for the launcher
^~~~~~~~~~~~~~~~~~~~~~~~~~