README: update text of button "Install"

The last step in instructions in the generated README.md file for
installing the plugin using IDE built-in plugin system is using the
button "Install Plugin".  However, this button's text is "Install", at
least as of IntelliJ IDEA 2023.1.  The text was "Install" since at least
the commit c254ba45cd17 ("IDEA-195193 Plugins: implement search",
2018-08-20) [1] where class InstallButton was introduced.  Since commit
ebb07b041133 ("i18n for plugins configurable, initial", 2020-01-27) [2]
the class uses bundle message "action.AnActionButton.text.install" from
IdeBundle [3].

Change the text of the button in the generated README.md file to
"Install" accordingly.

[1] c254ba45cd
[2] ebb07b0411
[3] ebb07b0411/platform/platform-resources-en/src/messages/IdeBundle.properties (L1714)

.github/template-cleanup/README.md

@@ -29,7 +29,7 @@ To keep everything working, do not remove `<!-- ... -->` sections.
 - Using IDE built-in plugin system:
   
   <kbd>Settings/Preferences</kbd> > <kbd>Plugins</kbd> > <kbd>Marketplace</kbd> > <kbd>Search for "%NAME%"</kbd> >
-  <kbd>Install Plugin</kbd>
+  <kbd>Install</kbd>
   
 - Manually: