Add a missing word in the documentation

Small patch adds the word 'to' to the documentation.

THANKS

@@ -95,6 +95,7 @@ Michiel De Wilde          [email protected]
 Mickael Labau             [email protected]
 Mike Castle               [email protected]
 Mike Sullivan             [email protected]
+Nate Guerin               [email protected]
 Neil Booth                [email protected]
 Nelson H. F. Beebe        [email protected]
 Nick Bowler               [email protected]

doc/bison.texi

@@ -8932,7 +8932,7 @@ clear the flag.
 
 Developing a parser can be a challenge, especially if you don't understand
 the algorithm (@pxref{Algorithm, ,The Bison Parser Algorithm}).  This
-chapter explains how understand and debug a parser.
+chapter explains how to understand and debug a parser.
 
 The first sections focus on the static part of the parser: its structure.
 They explain how to generate and read the detailed description of the