normalize $prefix to make it useable as schema

[RTLer]

hi
this controller uses graphql.prefix config as part of shema name but prefix can be some thing like api/graphql or something like it
i just normalize it to make it to search for api_graphql_schema instead of api/graphql_schema which isn't a valid route param name
for this instance graphql.php is :

[
    'prefix' => 'api/graphql',
    'routes' => '{api_graphql_schema?}',
]

[coveralls]

Coverage remained the same at 70.537% when pulling b6eafbd on RTLer:patch-1 into 66e9233 on Folkloreatelier:develop.