API Docs: Add missing project option shared_runners_enabled

Introduced into the API in e80e3f53

[ci skip]

doc/api/projects.md

@@ -424,6 +424,7 @@ Parameters:
 - `builds_enabled` (optional)
 - `wiki_enabled` (optional)
 - `snippets_enabled` (optional)
+- `shared_runners_enabled` (optional)
 - `public` (optional) - if `true` same as setting visibility_level = 20
 - `visibility_level` (optional)
 - `import_url` (optional)
@@ -447,6 +448,7 @@ Parameters:
 - `builds_enabled` (optional)
 - `wiki_enabled` (optional)
 - `snippets_enabled` (optional)
+- `shared_runners_enabled` (optional)
 - `public` (optional) - if `true` same as setting visibility_level = 20
 - `visibility_level` (optional)
 - `import_url` (optional)
@@ -472,6 +474,7 @@ Parameters:
 - `builds_enabled` (optional)
 - `wiki_enabled` (optional)
 - `snippets_enabled` (optional)
+- `shared_runners_enabled` (optional)
 - `public` (optional) - if `true` same as setting visibility_level = 20
 - `visibility_level` (optional)
 - `public_builds` (optional)