Fix docstring and max-specpdl-size warnings

[8dcc]

Changes:

• Replace single quote with backquote in docstring for graphviz-values-type-arrow. I didn't simply escape the quote because I saw the backquote form being used in other URLs.
• Replace obsolete max-specpdl-size variable with max-lisp-eval-depth, since the former is obsolete since 29.1. According to its documentation:

  To prevent runaway recursion, use max-lisp-eval-depth instead; it will indirectly limit the specpdl stack size as well.

Fixes the following warnings:

Warning (comp): graphviz-dot-mode.el:248:2: Warning: defvar `graphviz-values-type-arrow' docstring has wrong usage of unescaped single quotes (use \= or different quoting)
Warning (comp): graphviz-dot-mode.el:563:36: Warning: ‘max-specpdl-size’ is an obsolete variable (as of 29.1).
Warning (comp): graphviz-dot-mode.el:563:14: Warning: ‘max-specpdl-size’ is an obsolete variable (as of 29.1).

Sorry if I should have made a different PR for each commit.