Improve mesage on failed Graphviz optional (#11896)

* Improve mesage on failed Graphviz optional

I have recently seen several users struggle to install a complete
version of Graphviz upon seeing the `MissingOptionalLibraryError`
message.  This updates the message to make it clearer that the PyPI
distribution `graphviz` *is not* the actual Graphviz software, and it
must be installed separately.

* Tweak message

qiskit/utils/optionals.py

@@ -307,8 +307,12 @@
 
 HAS_GRAPHVIZ = _LazySubprocessTester(
     ("dot", "-V"),
-    name="graphviz",
-    install="'brew install graphviz' if on Mac, or by downloding it from their website",
+    name="Graphviz",
+    msg=(
+        "To install, follow the instructions at https://graphviz.org/download/."
+        " Qiskit needs the Graphviz binaries, which the 'graphviz' package on pip does not install."
+        " You must install the actual Graphviz software"
+    ),
 )
 HAS_PDFLATEX = _LazySubprocessTester(
     ("pdflatex", "-version"),