Fix extension name regex in issue template

Author: dweindl

.github/ISSUE_TEMPLATE/petab-extensions.md

@@ -9,7 +9,7 @@ assignees: ''
 ---
 
 **Name of the Extension**
-Please make sure that the extension name matches the regular expression `^[a-zA-Z_]\w*$`.
+Please make sure that the extension name matches the regular expression `^[a-zA-Z_][\w-]*$`.
 
 **Which problem would you like to address?**
 A clear and concise description of which use case you want to address and, if applicable, why the current specifications do not fulfill your requirements.
@@ -24,4 +24,4 @@ A clear and concise description in what way the proposed changes introduce featu
 A link to the website or github repository that accompanies the proposed extension.
 
 **List the toolboxes that support the proposed standard**
-A link to the website or github repository that contains the software that implements support for the standard.
+A link to the website or github repository that contains the software that implements support for the standard.