docs: single typo in 3rd sentence of the manual

Signed-off-by: Henri Menke <henri@henrimenke.de>

doc/generic/pgf/pgfmanual-en-introduction.tex

@@ -15,7 +15,7 @@ \section{Introduction}
 Tantau's) PhD thesis directly with pdf\LaTeX\ has now grown to become a
 full-blown graphics language with a manual of over a thousand pages. The wealth
 of options offered by \tikzname\ is often daunting to beginners; but
-fortunately this documentation comes with a number slowly-paced tutorials that
+fortunately this documentation comes with a number of slowly-paced tutorials that
 will teach you almost all you should know about \tikzname\ without your having
 to read the rest.