Fixed a doc build warning

Author: tleonhardt

docs/plugins/external_test.md

@@ -46,7 +46,7 @@ def example_app():
 
 ## Writing Tests
 
-Now write your tests that validate your application using the [cmd2_ext_test.ExternalTestMixin.app_cmd][] function to access the cmd2 application's commands. This allows invocation of the application's commands in the same format as a user would type. The results from calling a command matches what is returned from running an python script with cmd2's [run_pyscript](../features/builtin_commands.md#run_pyscript) command, which provides `stdout`, `stderr`, and the command's result data.
+Now write your tests that validate your application using the `cmd2_ext_test.ExternalTestMixin.app_cmd` function to access the cmd2 application's commands. This allows invocation of the application's commands in the same format as a user would type. The results from calling a command matches what is returned from running an python script with cmd2's [run_pyscript](../features/builtin_commands.md#run_pyscript) command, which provides `stdout`, `stderr`, and the command's result data.
 
 ```py
 from cmd2 import CommandResult

docs/testing.md

@@ -6,7 +6,7 @@ This covers special considerations when writing unit tests for a cmd2 applicatio
 
 ## Testing Commands
 
-The [External Test Plugin](plugins/external_test.md) provides a mixin class with an function that allows external calls to application commands. The `cmd2_ext_test.ExternalTestMixin.app_cmd()` function captures and returns stdout, stderr, and the command-specific result data.
+The [External Test Plugin](plugins/external_test.md) provides a mixin class with an function that allows external calls to application commands. The `cmd2_ext_test.ExternalTestMixin.app_cmd` function captures and returns stdout, stderr, and the command-specific result data.
 
 ## Mocking
 

pyproject.toml

@@ -37,6 +37,7 @@ dependencies = [
 [project.optional-dependencies]
 build = ["build", "setuptools", "setuptools-scm"]
 dev = [
+    "black",
     "codecov",
     "griffe-typingdoc",
     "invoke",
@@ -52,6 +53,7 @@ dev = [
     "twine",
 ]
 docs = [
+    "black",
     "griffe-typingdoc",
     "mkdocs-include-markdown-plugin",
     "mkdocs-macros-plugin",
@@ -275,6 +277,7 @@ packages = ["cmd2"]
 
 [tool.uv]
 dev-dependencies = [
+    "black",
     "build",
     "cmd2-ext-test",
     "codecov",