Make docs about expect_next_instance_of consistent

So we compare `expect_any_instance_of` with
`expect_next_instance_of`, but still mention
`allow_any_instance_of`.

doc/development/gotchas.md

@@ -92,7 +92,7 @@ describe API::Labels do
 end
 ```
 
-## Avoid using `allow_any_instance_of` in RSpec
+## Avoid using `expect_any_instance_of` or `allow_any_instance_of` in RSpec
 
 ### Why
 
@@ -102,7 +102,7 @@ end
   error like this:
 
     ```
-    1.1) Failure/Error: allow_any_instance_of(ApplicationSetting).to receive_messages(messages)
+    1.1) Failure/Error: expect_any_instance_of(ApplicationSetting).to receive_messages(messages)
          Using `any_instance` to stub a method (elasticsearch_indexing) that has been defined on a prepended module (EE::ApplicationSetting) is not supported.
     ```
 
@@ -112,7 +112,7 @@ Instead of writing:
 
 ```ruby
 # Don't do this:
-allow_any_instance_of(Project).to receive(:add_import_job)
+expect_any_instance_of(Project).to receive(:add_import_job)
 ```
 
 We could write: