Fix --show-snippet option consuming next CLI argument (#11684)

The show-snippet long option was defined with a single colon (required
value) instead of double colon (optional value), causing getopt() to
silently consume the next argument as its value.

Fixes #11683

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

Author: alies-dev

src/Psalm/Internal/Cli/Psalm.php

@@ -151,7 +151,7 @@ final class Psalm
         'root:',
         'set-baseline::',
         'show-info:',
-        'show-snippet:',
+        'show-snippet::',
         'stats',
         'threads:',
         'scan-threads:',