Fix unclosed parentheses error on "Create a widget to make buttons" code excerpt (#12877)

_Description of what this PR is changing or adding, and why:_
1. Stumbled on an unclosed parentheses error while doing the tutorial
2. Added the closing parenthesis
3. No more stumbling :D

_Issues fixed by this PR (if any):_
- Presumed none

_PRs or commits this PR depends on (if any):_
- Presumed none

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---------

Co-authored-by: Parker Lougheed <parlough@gmail.com>

Author: autorejecttop

examples/layout/lakes/step3/lib/main.dart

@@ -131,8 +131,6 @@ class ButtonWithText extends StatelessWidget {
       // #docregion button-section
     );
   }
-
-  // #enddocregion button-with-text
 }
 
-// #enddocregion button-section
+// #enddocregion button-with-text, button-section

src/content/ui/layout/tutorial.md

@@ -356,6 +356,7 @@ class ButtonWithText extends StatelessWidget {
       ],
     );
   }
+}
 ```
 
 ### Position the buttons with a `Row` widget
@@ -411,7 +412,6 @@ class ButtonWithText extends StatelessWidget {
       // ···
     );
   }
-
 }
 ```