fix typo

src/zc/buildout/buildout.txt

@@ -1009,7 +1009,7 @@ Extending sections (macros)
 
 A section (other than the buildout section) can extend one or more
 other sections using the ``<`` option.  Options from the referenced
-sections are copied to the refering section *before* variable
+sections are copied to the referring section *before* variable
 substitution.  This, together with the ability to refer to variables
 of the current section allows sections to be used as macros.