vela: Fixed formatting of SUPPORTED_OPS

mlplatform uses gitiles, which in turn renders markdown differently:
"There must be at least three hyphens in each column of the header row"
Updated the generation code and the snapshot file to respect this,
as well as changed the link from commonmark (which does not support
tables)

Signed-off-by: Michael McGeagh <michael.mcgeagh@arm.com>
Change-Id: If31860ce8e38ebe7d68bfec61faff805fc00345b
diff --git a/ethosu/vela/vela.py b/ethosu/vela/vela.py
index 7b76685..f03bae7 100644
--- a/ethosu/vela/vela.py
+++ b/ethosu/vela/vela.py
@@ -130,7 +130,8 @@
         "This file was automatically generated by Vela using the `--supported-ops-report` parameter.  ",
         f"Vela version: `{__version__}`",
         "",
-        "This file complies with [**CommonMark.**](https://commonmark.org)",
+        "This file complies with",
+        "[**Gitiles Markdown syntax**](https://github.com/google/gitiles/blob/master/Documentation/markdown.md)",
         "",
         "## Summary Table",
         "",
@@ -140,7 +141,7 @@
         "Please check the supported operator list for your chosen runtime for further information.",
         "",
         "| Operator | Constraints |",
-        "| - | - |",
+        "| --- | --- |",
     ]
     supported = SupportedOperators()
     op_constraint_links = []