Fix typo

Doc/ref/ref3.tex

@@ -910,7 +910,7 @@ imported modules are still available at the time when the
 \begin{methoddesc}[object]{__repr__}{self}
 Called by the \function{repr()}\bifuncindex{repr} built-in function
 and by string conversions (reverse quotes) to compute the ``official''
-string representation of an object.  It at all possible, this should
+string representation of an object.  If at all possible, this should
 look like a valid Python expression that could be used to recreate an
 object with the same value (given an appropriate environment).  If
 this is not possible, a string of the form \samp{<\var{...some useful