fix typo in doc (#12686)

Doc/extending/building.rst

@@ -145,7 +145,7 @@ that distutils gets the invocations right.
 Distributing your extension modules
 ===================================
 
-When an extension has been successfully build, there are three ways to use it.
+When an extension has been successfully built, there are three ways to use it.
 
 End-users will typically want to install the module, they do so by running ::
 
@@ -158,7 +158,7 @@ Module maintainers should produce source packages; to do so, they run ::
 In some cases, additional files need to be included in a source distribution;
 this is done through a :file:`MANIFEST.in` file; see :ref:`manifest` for details.
 
-If the source distribution has been build successfully, maintainers can also
+If the source distribution has been built successfully, maintainers can also
 create binary distributions. Depending on the platform, one of the following
 commands can be used to do so. ::