add missing "if"

Doc/lib/libfileinput.tex

@@ -77,7 +77,7 @@ The following functions use the global state created by
 \end{funcdesc}
 
 \begin{funcdesc}{isfirstline}{}
-  Returns true the line just read is the first line of its file,
+  Returns true if the line just read is the first line of its file,
   otherwise returns false.
 \end{funcdesc}