docs: correct -s command-line flag description

[DeepDiver1975]

What

Corrects the description of the -s / --show command-line flag in installing.adoc.

The docs claimed the flag "forces the settings page to be displayed on startup". Per the owncloud/client v7.1.0 source (src/gui/main.cpp:123-125), the flag actually:

Start with the main window visible, or if it is already running, bring it to the front. By default, the client launches in the background.

Why

The previous wording was inaccurate — there is no "settings page" behavior for this flag.

Closes #722

Companion PR onto the 7.1 branch: same change applied there.

🤖 Generated with Claude Code

[DeepDiver1975]

🤖 Automated code review by Claude Code review agent

Verdict: Approve. Verified the new wording against upstream owncloud/client at tag v7.1.0.

• src/gui/main.cpp:123-125 defines the option as {{"s"},{"show"}} with help text: "Start with the main window visible, or if it is already running, bring it to the front. By default, the client launches in the background." The PR's new description matches this verbatim, and documenting the --show long form is a nice addition.
• The old "forces the settings page to be displayed on startup" wording was inaccurate — there is no settings-page behavior tied to this flag's documented purpose. (For full transparency: -s does internally also route through slotShowSettings() and shares a hidden legacy --showsettings alias, but the option's own canonical help text is the main-window behavior, which is what end-user docs should reflect.)
• Scope docs-only (+1/-1); no broken AsciiDoc.

Pair consistency: Paired with #731 (7.1); identical change, baseRefName master. Consistent.