fix typo in github issue template

Except for the operating system on all other server configuration points the
colon is bold. The colon of the operating system wants to be bold, too.

Signed-off-by: Matthias Fritzsche <matthias@chemnitz.freifunk.net>

.github/ISSUE_TEMPLATE.md

@@ -10,7 +10,7 @@
 
 ### Server configuration
 
-**Operating system**:
+**Operating system:**
 
 **Web server:**