Update code in docs to be more consistent

Author: seratch

docs/_advanced/adapters.md

@@ -18,7 +18,7 @@ The full list adapters, as well as configuration and sample usage, can be found
 ```python
 from slack_bolt import App
 app = App(
-    signing_secret=os.environ.get("SIGNING_SECRET"),
+    signing_secret=os.environ.get("SLACK_SIGNING_SECRET"),
     token=os.environ.get("SLACK_BOT_TOKEN")
 )
 

docs/_advanced/ja_adapters.md

@@ -18,7 +18,7 @@ order: 0
 ```python
 from slack_bolt import App
 app = App(
-    signing_secret=os.environ.get("SIGNING_SECRET"),
+    signing_secret=os.environ.get("SLACK_SIGNING_SECRET"),
     token=os.environ.get("SLACK_BOT_TOKEN")
 )
 

docs/_basic/authenticating_oauth.md

@@ -35,7 +35,7 @@ oauth_settings = OAuthSettings(
 )
 
 app = App(
-    signing_secret=os.environ["SIGNING_SECRET"],
+    signing_secret=os.environ["SLACK_SIGNING_SECRET"],
     oauth_settings=oauth_settings
 )
 ```

docs/_basic/ja_authenticating_oauth.md

@@ -35,7 +35,7 @@ oauth_settings = OAuthSettings(
 )
 
 app = App(
-    signing_secret=os.environ["SIGNING_SECRET"],
+    signing_secret=os.environ["SLACK_SIGNING_SECRET"],
     oauth_settings=oauth_settings
 )
 ```