Insert missing pronoun

Doc/tut/glossary.tex

@@ -62,8 +62,8 @@ Python has builtin support for complex numbers, which are written with this
 latter notation; the imaginary part is written with a {}\code{j} suffix,
 e.g., {}\code{3+1j}.  To get access to complex equivalents of the
 {}\module{math} module, use {}\module{cmath}.  Use of complex numbers is a
-fairy advanced mathematical feature.  If you're not aware of a need for it's
-almost certain you can safely ignore them.
+fairy advanced mathematical feature.  If you're not aware of a need for them,
+it's almost certain you can safely ignore them.
 
 \index{descriptor}
 \item[descriptor]