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

[book]

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

• This set of changes does not require a perldelta entry.

[jkeenan]

Agreed.

$ grep -n -E '\{\^MAX_(EVAL_BEGIN_DEPTH|NESTED_EVAL_BEGIN_BLOCKS)}' ./pod/perlvar.pod 
604:=item ${^MAX_NESTED_EVAL_BEGIN_BLOCKS}
646:which makes it obvious why a ${^MAX_EVAL_BEGIN_DEPTH} of 1 would not

$ ack -l '\{\^MAX_NESTED_EVAL_BEGIN_BLOCKS}' .
INSTALL
pod/perl5380delta.pod
pod/perldiag.pod
pod/perlfunc.pod
pod/perltoc.pod
pod/perlvar.pod
t/op/eval.t
perl.h
op.c

$ ack -l '\{\^MAX_EVAL_BEGIN_DEPTH}' .
pod/perlvar.pod