docs(injector): API docs - remove lone code-block backticks (#11653)

The triple backticks in the markdown of the API entry are unbalanced.

modules/@angular/core/src/di/injector.ts

@@ -51,7 +51,6 @@ export abstract class Injector {
    * - Throws {@link NoProviderError} if no `notFoundValue` that is not equal to
    * Injector.THROW_IF_NOT_FOUND is given
    * - Returns the `notFoundValue` otherwise
-   * ```
    */
   get(token: any, notFoundValue?: any): any { return unimplemented(); }
 }