fix the variable name in the doc for ${^MAX_NESTED_EVAL_BEGIN_BLOCKS}

This seems to have existed since 741a5c7
(the initial commit), so it definitely looks like a typo that wasn't
spotted before.

Author: book

pod/perlvar.pod

@@ -643,8 +643,8 @@ fact the above code is equivalent to
     BEGIN { $n+=2 }
     BEGIN { $n+=1 }
 
-which makes it obvious why a ${^MAX_EVAL_BEGIN_DEPTH} of 1 would not
-block this code.
+which makes it obvious why a ${^MAX_NESTED_EVAL_BEGIN_BLOCKS} of 1
+would not block this code.
 
 Only C<BEGIN>'s executed inside of an C<eval> or C<require> (possibly via
 C<use>) are affected.