docs: fix typo in intro: "questions" => "question" (#1309)

Fix in context -- "question" should be singular:
<quote>
    I wish to start with the question “What is TikZ?”
</quote>

Signed-off-by: Alan D. Salewski <[email protected]>

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

@@ -19,7 +19,7 @@ \section{Introduction}
 will teach you almost all you should know about \tikzname\ without your having
 to read the rest.
 
-I wish to start with the questions ``What is \tikzname?'' Basically, it just
+I wish to start with the question ``What is \tikzname?'' Basically, it just
 defines a number of \TeX\ commands that draw graphics. For example, the code
 |\tikz \draw (0pt,0pt) -- (20pt,6pt);| yields the line \tikz \draw (0pt,0pt) --
 (20pt,6pt); and the code |\tikz \fill[orange] (1ex,1ex) circle (1ex);| yields