chore(python): fix column sizing issue in docs (#884)

Author: busunkim96

synthtool/gcp/templates/python_library/docs/_static/custom.css

@@ -1,4 +1,9 @@
 div#python2-eol {
 	border-color: red;
 	border-width: medium;
-} 
+} 
+
+/* Ensure minimum width for 'Parameters' / 'Returns' column */
+dl.field-list > dt {
+    min-width: 100px
+}