Typo.

Doc/ext/building.tex

@@ -8,7 +8,7 @@ file for building make files for building dynamically-linked
 extensions and custom interpreters.  Starting with Python 2.0, this
 mechanism (known as related to Makefile.pre.in, and Setup files) is no
 longer supported. Building custom interpreters was rarely used, and
-extensions modules can be built using distutils.
+extension modules can be built using distutils.
 
 Building an extension module using distutils requires that distutils
 is installed on the build machine, which is included in Python 2.x and