Fix non-ASCII characters in V3Covergroup.cpp comment

Replace Unicode em-dash and curly apostrophe with plain ASCII
equivalents to pass t_dist_whitespace CI check.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

src/V3Covergroup.cpp

@@ -1342,7 +1342,7 @@ class FunctionalCoverageVisitor final : public VNVisitor {
                 }
 
                 if (!foundCpp) {
-                    // Name not found as an explicit coverpoint — it's likely a direct variable
+                    // Name not found as an explicit coverpoint - it's likely a direct variable
                     // reference (implicit coverpoint). Silently ignore; cross is dropped.
                     UINFO(4, "  Ignoring cross with implicit variable reference: " << refp->name()
                                                                                    << endl);